feat(studio): Welcome page — Phase A scaffolding (v0.37.0)

First slice of the new sidebar destination outlined in
docs/landing-page-design.md. Static content only — embedded
docs (Phase B) and the live getting-started checklist
(Phase C) follow.

Phase A:

- New `WelcomePage` at `lib/pages/welcome.dart`, registered
  as sidebar slot 0 above Doctor. Default selectedIndex stays
  0, so a fresh launch lands on Welcome.
- Hero card: gradient backdrop matching the Today-Hero in the
  store, F∆I Platform headline, subtitle taken from CLAUDE.md
  ("deterministic workflow engine for AI-assisted document
  processing in regulated environments").
- Three-pillar row "Hub / Module / Flow" — operator-readable
  prose, not architecture-doc dense. Stacks to a column under
  640 dp window width so card text never gets cropped.
- Trust-posture deck — "Sandbox by default", "Tamper-evident
  audit log", "Air-gap ready". Same content that used to
  rotate as carousel slides in the store; reading them as a
  single deck with full prose works better than rotating
  through fragments.
- AppBar follows the same `titleSpacing: FaiSpace.xl`
  alignment as the Store so the title sits flush with the
  body padding.
- 13 new ARB keys for the page content (EN + DE).

Smoke test now expects "Welcome" in the navigation rail; the
existing `Doctor / Store / Flows / Audit / Approvals`
expectations stay unchanged.

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

lib/main.dart

@@ -17,6 +17,7 @@ import 'pages/audit.dart';
 import 'pages/doctor.dart';
 import 'pages/flows.dart';
 import 'pages/store.dart';
+import 'pages/welcome.dart';
 import 'theme/theme.dart';
 import 'theme/tokens.dart';
 import 'widgets/fai_search_palette.dart';
@@ -25,7 +26,7 @@ import 'widgets/widgets.dart';
 /// Studio's own build version. Bump on every UI commit so the
 /// running app self-identifies — visible in the sidebar header
 /// and quick-glance proof that you're seeing the current build.
-const String kStudioVersion = '0.36.0';
+const String kStudioVersion = '0.37.0';
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
@@ -139,6 +140,12 @@ class _StudioShellState extends State<StudioShell> {
   Timer? _healthPoll;
 
   static const _pages = <_NavPage>[
+    _NavPage(
+      id: 'welcome',
+      icon: Icons.waving_hand_outlined,
+      selectedIcon: Icons.waving_hand,
+      page: WelcomePage(),
+    ),
     _NavPage(
       id: 'doctor',
       icon: Icons.health_and_safety_outlined,
@@ -752,6 +759,8 @@ class _NavPage {
     final l = AppLocalizations.of(context);
     if (l == null) return id;
     switch (id) {
+      case 'welcome':
+        return l.navWelcome;
       case 'doctor':
         return l.navDoctor;
       case 'modules':