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/friendly_error.dart

@@ -26,12 +26,27 @@ class FriendlyError {
     required this.detail,
     this.hint,
   });
+
+  // The on-disk error log serialises thrown objects via toString —
+  // keep the whole story (headline, hint, verbatim detail) in one
+  // readable record.
+  @override
+  String toString() => [
+    headline,
+    ?hint,
+    if (detail.isNotEmpty && detail != headline) detail,
+  ].join('\n');
 }
 
 /// Map an arbitrary thrown object to a [FriendlyError]. Always
 /// returns a value — never throws — so callers can drop the
 /// result straight into UI without try/catch ceremony.
 FriendlyError friendlyError(Object error, AppLocalizations l) {
+  // A pre-built FriendlyError passes through unchanged — call
+  // sites that already know the precise story (e.g. the setup
+  // wizard's CLI-version-skew case) construct one directly and
+  // still route through the shared presentation helpers.
+  if (error is FriendlyError) return error;
   // We deliberately don't import package:grpc here so Studio
   // doesn't have to add it to its own pubspec — the dependency
   // lives one layer down in chain_client_sdk. `GrpcError` has a