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>

test/load_docs_test.dart

@@ -0,0 +1,52 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart' show rootBundle;
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter_markdown_plus/flutter_markdown_plus.dart';
+import 'package:flutter_localizations/flutter_localizations.dart';
+import 'package:fai_studio/l10n/app_localizations.dart';
+import 'package:fai_studio/theme/theme.dart';
+
+/// Recreates the exact DocReaderSheet code path that
+/// welcome.dart uses, including FaiTheme.markdownStyle, so a
+/// regression in either rootBundle, the markdown parser, or
+/// the style sheet surfaces here instead of only at runtime
+/// when an operator clicks a doc card.
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  testWidgets('DocReaderSheet renders all four german + english docs',
+      (tester) async {
+    final theme = FaiTheme.light();
+    for (final slug in ['architecture', 'security', 'audit', 'flows']) {
+      for (final locale in ['de', 'en']) {
+        final path = locale == 'de'
+            ? 'assets/docs/${slug}_de.md'
+            : 'assets/docs/$slug.md';
+        final text = await rootBundle.loadString(path);
+
+        await tester.pumpWidget(MaterialApp(
+          locale: Locale(locale),
+          supportedLocales: const [Locale('en'), Locale('de')],
+          localizationsDelegates: const [
+            AppLocalizations.delegate,
+            GlobalMaterialLocalizations.delegate,
+            GlobalWidgetsLocalizations.delegate,
+            GlobalCupertinoLocalizations.delegate,
+          ],
+          theme: theme,
+          home: Scaffold(
+            body: Markdown(
+              data: text,
+              padding: const EdgeInsets.all(8),
+              selectable: true,
+              styleSheet: FaiTheme.markdownStyle(theme),
+            ),
+          ),
+        ));
+        await tester.pump();
+        // ignore: avoid_print
+        print('rendered $path');
+      }
+    }
+  });
+}