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/l10n/app_localizations_en.dart

@@ -11,6 +11,64 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get appTitle => 'F∆I Studio';
 
+  @override
+  String get navWelcome => 'Welcome';
+
+  @override
+  String get welcomeHeroTitle => 'F∆I Platform';
+
+  @override
+  String get welcomeHeroSubtitle =>
+      'Deterministic workflow engine for AI-assisted document processing in regulated environments.';
+
+  @override
+  String get welcomePillarsHeader => 'HOW IT FITS TOGETHER';
+
+  @override
+  String get welcomePillarHubTitle => 'Hub';
+
+  @override
+  String get welcomePillarHubBody =>
+      'One Rust binary that loads modules and runs flows. SQLite for state. No Docker, no message broker, no background services to babysit.';
+
+  @override
+  String get welcomePillarModuleTitle => 'Module';
+
+  @override
+  String get welcomePillarModuleBody =>
+      'A small WebAssembly component that does one thing. Declares the network endpoints, files, and environment variables it touches; the hub enforces the list.';
+
+  @override
+  String get welcomePillarFlowTitle => 'Flow';
+
+  @override
+  String get welcomePillarFlowBody =>
+      'A YAML file that chains modules together. Inputs go in, modules run in declared order, outputs come out — every step recorded in a hash-chained audit log.';
+
+  @override
+  String get welcomeTrustHeader => 'TRUST POSTURE';
+
+  @override
+  String get welcomeTrustSandboxTitle => 'Sandbox by default';
+
+  @override
+  String get welcomeTrustSandboxBody =>
+      'Every module ships with an explicit permission list — network endpoints, files, environment variables. The hub enforces it; nothing reaches outside the sandbox without operator approval.';
+
+  @override
+  String get welcomeTrustAuditTitle => 'Tamper-evident audit log';
+
+  @override
+  String get welcomeTrustAuditBody =>
+      'Flow runs, install / uninstall actions, approval decisions — all of them write into a hash-chained audit log. The Doctor page verifies the chain end-to-end on every load.';
+
+  @override
+  String get welcomeTrustAirgapTitle => 'Air-gap ready';
+
+  @override
+  String get welcomeTrustAirgapBody =>
+      'The whole hub fits in a single binary that runs on Linux, macOS and Windows. Once a module is installed, the flow it powers runs without further network access — perfect for regulated environments.';
+
   @override
   String get navDoctor => 'Doctor';