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/data/system_actions.dart

@@ -18,6 +18,7 @@
 
 import 'dart:io';
 
+import 'package:meta/meta.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 /// Sentinel returned in `_runFai(...).stderr` when no `fai`
@@ -73,6 +74,28 @@ class SystemActions {
   /// binary / read the install guide" recovery path.
   static bool chainBinaryExists() => _faiExecutable() != null;
 
+  /// Test seam: when set, [_runFai] returns this function's result
+  /// instead of spawning a real process, and [resolvedChainBinary] /
+  /// [chainBinaryExists] answer from [debugResolveOverride]. Lets
+  /// widget tests drive the CLI error paths deterministically.
+  @visibleForTesting
+  static Future<({bool ok, String stdout, String stderr})> Function(
+    List<String> args,
+  )?
+  debugRunFaiOverride;
+
+  /// Test seam companion to [debugRunFaiOverride]: overrides binary
+  /// resolution (may return null to simulate "no binary found").
+  @visibleForTesting
+  static String? Function()? debugResolveOverride;
+
+  /// Absolute path of the `chain` binary Studio would execute right
+  /// now, or null when none can be located. Public so surfaces that
+  /// are about to spawn the binary (the guided-setup wizard) can say
+  /// WHAT they will run — before macOS asks the operator for folder
+  /// permission because of where that binary happens to live.
+  static String? resolvedChainBinary() => _faiExecutable();
+
   /// Ask the OS to open [path] in the default handler. On macOS
   /// this opens text files in TextEdit, configs in the registered
   /// editor, etc. Returns true on a clean spawn (process exited
@@ -219,6 +242,8 @@ class SystemActions {
   static Future<({bool ok, String stdout, String stderr})> _runFai(
     List<String> args,
   ) async {
+    final runOverride = debugRunFaiOverride;
+    if (runOverride != null) return runOverride(args);
     final exe = _faiExecutable();
     if (exe == null) {
       // Sentinel, not a user-facing string. Callers detect this
@@ -249,6 +274,8 @@ class SystemActions {
   /// the file picker), $CHAIN_BIN, PATH, fallback to the canonical
   /// install location under the user's home dir.
   static String? _faiExecutable() {
+    final resolveOverride = debugResolveOverride;
+    if (resolveOverride != null) return resolveOverride();
     final override = _faiBinaryOverride;
     if (override != null &&
         override.isNotEmpty &&