feat(studio): Welcome page Phase B — embedded doc reader (v0.38.0)

Second slice of the Welcome surface. Operator-facing
documentation now lives inside Studio as bundled assets and
renders inline via a modal sheet — no browser, no external
link, air-gap-tauglich.

- Four operator-readable explainers under `assets/docs/`,
  each with an EN + DE pair:
    architecture[_de].md  — Hub / Module / Flow + how they fit
    security[_de].md       — sandbox model, declared perms,
                              operator ceiling
    audit[_de].md          — hash-chained log, WORM-1 mechanics,
                              `fai admin verify-events`
    flows[_de].md          — flow YAML, templating reference,
                              extract→summarize example
  These are short (≈ 300-500 words each), operator-shaped
  prose. Not copies of the architecture docs in
  fai_platform/docs/architecture/ — those are
  contributor-dense.

- pubspec.yaml declares `assets/docs/` so the markdown ships
  inside the Studio binary. Air-gap deployments read them
  with no network access.

- New `_DocReaderSheet` modal: 85 %-of-viewport bottom sheet,
  drag handle + title bar with the doc icon and close button,
  scrollable Markdown body styled to match Studio chrome.
  Loads `assets/docs/<slug>_<locale>.md` first, falls back to
  the EN file. Locale comes from
  `Localizations.localeOf(context)`.

- `_DocsRow` on the Welcome page sits below the trust-posture
  deck. Two-column grid on ≥ 640 dp, single column below.
  Each card is icon + title + one-line blurb + chevron;
  click opens the reader sheet for that slug.

- 12 new ARB keys for the docs section (header, blurb, four
  card titles + blurbs, close button, error message).
- 4 new icons reused: `account_tree_outlined` (architecture),
  `shield_outlined` (security), `verified_outlined` (audit),
  `alt_route_outlined` (flows).

Implements Phase B of `docs/landing-page-design.md`. Phase C
(live getting-started checklist with persistent state) is the
last remaining slice.

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

lib/l10n/app_localizations_de.dart

@@ -69,6 +69,49 @@ class AppLocalizationsDe extends AppLocalizations {
   String get welcomeTrustAirgapBody =>
       'Der gesamte Hub steckt in einem einzigen Binary für Linux, macOS und Windows. Sobald ein Modul installiert ist, läuft der Flow, der es nutzt, ohne weiteren Netzwerkzugriff — ideal für regulierte Umgebungen.';
 
+  @override
+  String get welcomeDocsHeader => 'DOKUMENTATION';
+
+  @override
+  String get welcomeDocsBody =>
+      'Operator-Erklärungen, inline gerendert. Kein Browser, kein externer Link — air-gap-tauglich.';
+
+  @override
+  String get welcomeDocArchitectureTitle => 'Architektur in einem Absatz';
+
+  @override
+  String get welcomeDocArchitectureBlurb =>
+      'Hub, Modul, Flow — und wie die drei zusammenpassen.';
+
+  @override
+  String get welcomeDocSecurityTitle => 'Sandbox-Modell';
+
+  @override
+  String get welcomeDocSecurityBlurb =>
+      'Was ein Modul deklariert, was der Operator deckelt, was der Hub durchsetzt.';
+
+  @override
+  String get welcomeDocAuditTitle => 'Manipulationssicheres Audit-Log';
+
+  @override
+  String get welcomeDocAuditBlurb =>
+      'Wie die Hash-Kette funktioniert und warum WORM-1 für KRITIS reicht.';
+
+  @override
+  String get welcomeDocFlowsTitle => 'Flow-Komposition';
+
+  @override
+  String get welcomeDocFlowsBlurb =>
+      'YAML-Grundlagen, Templating-Referenz, das Extract→Summarize-Beispiel.';
+
+  @override
+  String get welcomeDocClose => 'Schließen';
+
+  @override
+  String welcomeDocFailedToLoad(String error) {
+    return 'Doku konnte nicht geladen werden: $error';
+  }
+
   @override
   String get navDoctor => 'Diagnose';