fix(welcome): structured doc-load error + Forgejo fallback link

The doc-reader bottom-sheet in welcome.dart hits rootBundle
to load assets/docs/<slug>[_de].md. When the asset isn't in
the bundle (e.g. the binary on disk predates the
flutter_markdown_plus migration in 69b54629 or an in-progress
asset reorganisation), the FutureBuilder's error branch dumps
the raw PlatformException text into FaiErrorBox with no
hint at what the operator should do.

Switch the throw to a structured `_DocReaderError` that
carries:
  - the slug, locale, and both attempted asset paths
  - the underlying error message
  - a precomputed `forgejoUrl` pointing at the same doc on
    the platform repo's main branch

The error UI gains a TextButton.icon below FaiErrorBox that
opens the Forgejo copy in the OS browser via the existing
SystemActions.openInOs path — operators always have a
working out, even when the local bundle is broken.

The error toString() also surfaces "rebuild Studio" as the
top suggestion, since the most common cause for asset
mismatches is a stale binary running against a newer source
tree.

New regression test (test/load_docs_test.dart) pumps the
exact code path the sheet uses (rootBundle.loadString +
Markdown with FaiTheme.markdownStyle) against all eight asset
paths (architecture/security/audit/flows × en/de). Catches a
future bundle drift before an operator notices.

No behavioural change on the happy path. Both the rendered
markdown widget and the closed-sheet animation are unchanged.

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

lib/pages/welcome.dart

@@ -910,6 +910,36 @@ class _DocCard extends StatelessWidget {
   }
 }
 
+/// Structured error type for the doc reader. Allows the
+/// error UI to render distinct lines for "what we tried",
+/// "what to try next", and "the raw underlying error" — and
+/// to surface the Forgejo browser fallback as a one-click
+/// link instead of a sentence to copy.
+class _DocReaderError {
+  final String slug;
+  final String locale;
+  final List<String> attempted;
+  final String underlying;
+  const _DocReaderError({
+    required this.slug,
+    required this.locale,
+    required this.attempted,
+    required this.underlying,
+  });
+
+  String get forgejoUrl =>
+      'https://git.flemming.ai/fai/platform/src/branch/main/docs/architecture/$slug.md';
+
+  @override
+  String toString() =>
+      'Could not load the bundled doc "$slug" ($locale).\n'
+      'Tried: ${attempted.join(", ")}\n'
+      'Underlying: $underlying\n'
+      'This typically means the Studio binary on disk is from\n'
+      'before the doc bundle changed. Rebuild Studio (fai studio\n'
+      'or the build pipeline) or open the Forgejo copy in browser.';
+}
+
 /// Modal bottom-sheet that loads the markdown for a doc entry
 /// from `assets/docs/<slug>[_de].md` and renders it inline via
 /// flutter_markdown. No browser, no network — KRITIS-friendly.
@@ -951,18 +981,24 @@ class _DocReaderSheetState extends State<_DocReaderSheet> {
     final locale = Localizations.localeOf(context).languageCode;
     final localised = 'assets/docs/${widget.entry.slug}_$locale.md';
     final fallback = 'assets/docs/${widget.entry.slug}.md';
+    // Cache-bust attempts. If the operator is running a stale
+    // build from before the flutter_markdown_plus migration,
+    // the .dart_tool asset manifest can be inconsistent with
+    // the compiled binary — surface "rebuild Studio" as a
+    // first-class hint rather than just dumping the raw
+    // PlatformException string into FaiErrorBox.
     try {
       return await rootBundle.loadString(localised);
     } catch (firstErr) {
       try {
         return await rootBundle.loadString(fallback);
       } catch (secondErr) {
-        // Surface BOTH attempted paths and the underlying
-        // errors so the operator can copy a useful diagnostic
-        // out of FaiErrorBox without us having to ship a doc
-        // about reading Flutter asset errors.
-        throw 'tried "$localised": $firstErr\n'
-            'then "$fallback": $secondErr';
+        throw _DocReaderError(
+          slug: widget.entry.slug,
+          locale: locale,
+          attempted: [localised, fallback],
+          underlying: '$firstErr / $secondErr',
+        );
       }
     }
   }
@@ -1028,6 +1064,9 @@ class _DocReaderSheetState extends State<_DocReaderSheet> {
                   );
                 }
                 if (snap.hasError) {
+                  final err = snap.error;
+                  final structured =
+                      err is _DocReaderError ? err : null;
                   return Padding(
                     padding: const EdgeInsets.all(FaiSpace.xxl),
                     child: Column(
@@ -1041,10 +1080,19 @@ class _DocReaderSheetState extends State<_DocReaderSheet> {
                         ),
                         const SizedBox(height: FaiSpace.sm),
                         FaiErrorBox(
-                          error: snap.error,
+                          error: err,
                           isError: true,
                           maxHeight: 200,
                         ),
+                        if (structured != null) ...[
+                          const SizedBox(height: FaiSpace.md),
+                          TextButton.icon(
+                            icon: const Icon(Icons.open_in_new, size: 16),
+                            label: Text(structured.forgejoUrl),
+                            onPressed: () =>
+                                SystemActions.openInOs(structured.forgejoUrl),
+                          ),
+                        ],
                       ],
                     ),
                   );