fix(setup): wizard errors copyable above the dialog, CLI skew explained, exec transparency

Field test of the setup wizard surfaced three trust breaks in one run:
an unexplained macOS Documents permission prompt, a perceived crash,
and an error message whose copy button could not be reached.

Root causes and fixes:
- chain init failures were shown as a SnackBar, which lands BEHIND the
  wizard's modal barrier: dimmed, clipped, copy unreachable — and the
  click aimed at it hit the barrier, dismissing the whole wizard with
  all answers (the perceived crash). Errors now open a modal dialog
  ABOVE the wizard via showChainErrorDialog with a copyable detail
  block, and the wizard is no longer barrier-dismissible.
- When the resolved chain binary is older than Studio and rejects
  --plan-json, the wizard now explains the version skew in plain
  language (binary path + update path) instead of leaking a raw clap
  usage error. A missing binary gets its own localized story.
- Step 3 announces which chain binary the preview will execute; when
  that binary physically lives (symlinks resolved) in a TCC-protected
  folder, the wizard pre-explains the macOS folder prompt.

Supporting changes: FriendlyError passes through friendlyError()
unchanged so call sites can ship precise localized stories through the
shared presentation; SystemActions gains resolvedChainBinary() plus
run/resolve test seams; ChainErrorBox hugs its content instead of
filling an unbounded dialog; the wizard's answers file is written
synchronously (the async dart:io variants never complete under the
widget-test fake-async zone).

Verified: flutter analyze clean, 53 tests green (6 new wizard error-
path tests incl. clipboard round-trip), plus a live GUI walk on macOS
in dark + light with a stale binary (skew dialog, copy verified via
clipboard) and with the real binary (TCC pre-explanation with the
resolved path, full plan preview).

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/widgets/guided_setup_dialog.dart

@@ -16,6 +16,7 @@ import 'dart:io';
 import 'package:flutter/material.dart';
 
 import '../data/error_presentation.dart';
+import '../data/friendly_error.dart';
 import '../data/hub.dart';
 import '../data/system_actions.dart';
 import '../l10n/app_localizations.dart';
@@ -118,8 +119,14 @@ class GuidedSetupDialog extends StatefulWidget {
 
   static Future<void> show(BuildContext context) {
     final shell = StudioShellState.of(context);
+    // Not barrier-dismissible: a stray click outside the dialog must
+    // not throw away a half-answered wizard (and error SnackBars used
+    // to lure exactly that click — the operator aimed for the message
+    // below the barrier and lost everything). Leaving is explicit:
+    // Abbrechen on step 1, Zurück everywhere else.
     return showDialog<void>(
       context: context,
+      barrierDismissible: false,
       builder: (_) => GuidedSetupDialog(shell: shell),
     );
   }
@@ -276,12 +283,15 @@ class _GuidedSetupDialogState extends State<GuidedSetupDialog> {
   /// racy on multi-user machines. Cleaned up in [dispose].
   Directory? _answersDir;
 
-  Future<String> _writeAnswers() async {
-    _answersDir ??= await Directory.systemTemp.createTemp('chain-setup-');
+  String _writeAnswers() {
+    // Sync on purpose: the file is a handful of lines, and the async
+    // dart:io variants never complete inside the fake-async zone
+    // widget tests run in — the wizard would hang there forever.
+    _answersDir ??= Directory.systemTemp.createTempSync('chain-setup-');
     final f = File(
       '${_answersDir!.path}${Platform.pathSeparator}answers.yaml',
     );
-    await f.writeAsString(_answersYaml());
+    f.writeAsStringSync(_answersYaml());
     return f.path;
   }
 
@@ -299,7 +309,7 @@ class _GuidedSetupDialogState extends State<GuidedSetupDialog> {
       _busy = true;
       _plan = null;
     });
-    final path = await _writeAnswers();
+    final path = _writeAnswers();
     final r = await SystemActions.chainInit(['--answers', path, '--plan-json']);
     if (!mounted) return;
     setState(() => _busy = false);
@@ -311,13 +321,85 @@ class _GuidedSetupDialogState extends State<GuidedSetupDialog> {
           _step = _totalSteps; // review
         });
       } catch (_) {
-        showFaiProcessError(context, 'chain init --plan-json', r.stdout, r.stderr);
+        await _showCliError('chain init --plan-json', r.stdout, r.stderr);
       }
     } else {
-      showFaiProcessError(context, 'chain init --plan-json', r.stdout, r.stderr);
+      await _showCliError('chain init --plan-json', r.stdout, r.stderr);
     }
   }
 
+  /// Surface a failed `chain init` call as a modal dialog stacked
+  /// ABOVE this wizard. A SnackBar would land BEHIND the wizard's
+  /// modal barrier: visible but dimmed, its copy button unreachable —
+  /// and the operator's attempt to click it hits the barrier instead.
+  /// Known failure shapes get a localized plain-language headline;
+  /// the verbatim CLI output stays copyable behind "Details".
+  Future<void> _showCliError(String source, String stdout, String stderr) {
+    final l = AppLocalizations.of(context)!;
+    final raw = [
+      stderr.trim(),
+      stdout.trim(),
+    ].where((s) => s.isNotEmpty).join('\n\n');
+    final bin = SystemActions.resolvedChainBinary();
+    final FriendlyError friendly;
+    if (stderr.trim() == kFaiBinaryNotFound) {
+      // Sentinel, never shown verbatim (see SystemActions docs).
+      friendly = FriendlyError(
+        headline: l.setupNoBinary,
+        detail: '',
+        hint: l.setupNoBinaryHint,
+      );
+    } else if (raw.contains('unexpected argument')) {
+      // Version skew: this Studio speaks a newer `chain init` dialect
+      // than the binary it found (e.g. a stale build behind a channel
+      // symlink). Name the binary and the way out instead of leaking
+      // a raw clap usage error.
+      friendly = FriendlyError(
+        headline: l.setupCliTooOld,
+        detail: raw,
+        hint: l.setupCliTooOldHint(bin ?? 'chain'),
+      );
+    } else {
+      friendly = FriendlyError(
+        headline: source == 'chain init --apply'
+            ? l.setupApplyFailed
+            : l.setupPreviewFailed,
+        detail: raw.isEmpty ? 'process exited non-zero (no output)' : raw,
+        hint: bin == null ? null : l.setupCliUsedBinary(bin),
+      );
+    }
+    return showChainErrorDialog(context, source, friendly);
+  }
+
+  /// Physical location of the resolved `chain` binary (symlinks
+  /// followed) — the path that decides whether macOS shows a folder-
+  /// permission prompt when Studio executes it, regardless of how
+  /// harmless the symlink's own path looks.
+  String? _physicalChainBinary() {
+    final bin = SystemActions.resolvedChainBinary();
+    if (bin == null) return null;
+    try {
+      return File(bin).resolveSymbolicLinksSync();
+    } on FileSystemException {
+      return bin;
+    }
+  }
+
+  /// True when executing the resolved binary can trigger a macOS
+  /// folder-permission (TCC) prompt because it physically lives in a
+  /// protected folder. The wizard says so BEFORE the first exec —
+  /// an unexplained "Studio wants access to Documents" prompt in the
+  /// middle of setup reads as a trust break.
+  bool _binaryNeedsFolderPermission() {
+    if (!Platform.isMacOS) return false;
+    final physical = _physicalChainBinary();
+    final home = Platform.environment['HOME'];
+    if (physical == null || home == null || home.isEmpty) return false;
+    return physical.startsWith('$home/Documents/') ||
+        physical.startsWith('$home/Desktop/') ||
+        physical.startsWith('$home/Downloads/');
+  }
+
   /// Warning lines the apply emitted on success (e.g. the empty
   /// trusted_publishers caveat). Swallowing them made the wizard
   /// claim more than the config delivers — show them instead.
@@ -325,7 +407,7 @@ class _GuidedSetupDialogState extends State<GuidedSetupDialog> {
 
   Future<void> _apply() async {
     setState(() => _busy = true);
-    final path = await _writeAnswers();
+    final path = _writeAnswers();
     final r = await SystemActions.chainInit(
       ['--answers', path, '--apply', '--force'],
     );
@@ -343,7 +425,7 @@ class _GuidedSetupDialogState extends State<GuidedSetupDialog> {
       });
       unawaited(_probeHub());
     } else {
-      showFaiProcessError(context, 'chain init --apply', r.stdout, r.stderr);
+      await _showCliError('chain init --apply', r.stdout, r.stderr);
     }
   }
 
@@ -527,6 +609,18 @@ class _GuidedSetupDialogState extends State<GuidedSetupDialog> {
             value: _dataLocal,
             onChanged: (v) => setState(() => _dataLocal = v),
           ),
+          // Exec transparency: the next click runs the `chain` binary.
+          // Say which one — and warn when its physical location will
+          // make macOS ask for folder access, so the prompt (if any)
+          // arrives explained instead of as a trust break.
+          const SizedBox(height: ChainSpace.sm),
+          if (!SystemActions.chainBinaryExists())
+            _hintRow(l.setupNoBinaryStepHint)
+          else ...[
+            _hintRow(l.setupExecHint(SystemActions.resolvedChainBinary()!)),
+            if (_binaryNeedsFolderPermission())
+              _hintRow(l.setupExecHintTcc(_physicalChainBinary()!)),
+          ],
         ],
         // The free-text alternative lives on the first step: describe
         // the goal, the system AI pre-selects the menu answers. Menu