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/l10n/app_localizations_en.dart

@@ -3234,6 +3234,50 @@ class AppLocalizationsEn extends AppLocalizations {
   String get setupFreeTextUnavailable =>
       'The free-text path needs a configured system AI (Settings → System AI). The choices above always work — no AI required.';
 
+  @override
+  String setupExecHint(String path) {
+    return 'For the preview and setup, Studio runs the “chain” program: $path';
+  }
+
+  @override
+  String setupExecHintTcc(String path) {
+    return 'The program lives at $path — macOS may ask once for access to that folder (e.g. “Documents”). That is expected and fine.';
+  }
+
+  @override
+  String get setupNoBinaryStepHint =>
+      'The “chain” program could not be found on this machine — the preview in the next step will fail. You can pick it under “Doctor”, or install Ch∆In.';
+
+  @override
+  String get setupNoBinary =>
+      'The “chain” program could not be found on this machine.';
+
+  @override
+  String get setupNoBinaryHint =>
+      'Without it the assistant cannot apply the setup. Pick it under “Doctor”, or reinstall Ch∆In.';
+
+  @override
+  String get setupCliTooOld =>
+      'The installed “chain” program is older than Studio and does not know this assistant yet.';
+
+  @override
+  String setupCliTooOldHint(String path) {
+    return 'Found: $path. Please update Ch∆In (“Doctor” page → update, or in a terminal: chain update apply) and reopen the assistant afterwards.';
+  }
+
+  @override
+  String get setupPreviewFailed =>
+      'The plan preview could not be created — the “chain” program reported an error.';
+
+  @override
+  String get setupApplyFailed =>
+      'The setup could not be applied — the “chain” program reported an error.';
+
+  @override
+  String setupCliUsedBinary(String path) {
+    return 'Executed: $path';
+  }
+
   @override
   String get setupFreeTextParseError =>
       'The system AI did not return a usable suggestion. Pick from the menu above — or try again.';