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>

test/widget_test.dart

@@ -16,6 +16,9 @@ void main() {
     );
 
     expect(find.text('F∆I Studio'), findsOneWidget);
+    // Welcome tab added in v0.37.0 — first surface a new
+    // operator sees.
+    expect(find.text('Welcome'), findsWidgets);
     expect(find.text('Doctor'), findsWidgets);
     // Modules tab dropped in v0.35.0 — info now lives inside
     // the Store detail sheet.