feat(app): loading, error and not-found states for the user-facing routes

[jdjioe5-cpu]

Summary

Closes #37. Fulfills Roadmap row M3-27 (Phase 4, M3 App, Priority P1).

This PR introduces route-level designed states across user-facing routes:

• Shared Primitives: Implemented Spinner and StatePanel in components/ui/ with role="status"|"alert"|"region", accessibility attributes, and motion-safe:animate-spin respecting prefers-reduced-motion.
• Loading States: Added root app/loading.tsx as well as route-specific loading states for app/locks/loading.tsx ("Reading the lock from the network…") and app/payee/loading.tsx ("Checking verified payees…").
• Error State: Added app/error.tsx client component with autofocus on the retry action, maintaining reset() behavior, with zero technical traces or stack leaks in the UI.
• Not-Found State: Added app/not-found.tsx with honest wording distinguishing between local absence and indexer lag.
• Strings & Roadmap: All new strings externalized to messages/en.json. Marked M3-27 as DONE in ROADMAP.md with Changelog entry.

Hard Rules / Invariants Touched

• Hard Rule 5: Fragment privacy preserved. app/claim/** is untouched; error.tsx never logs, exposes or references claim-link fragments or addresses.
• Hard Rule 7: Honest wording. not-found.tsx explicitly notes indexer propagation delay rather than claiming on-chain absence.
• AGENTS.md §8.3: Mobile-first and slow connection friendly; zero inline text in JSX (verified by lib/i18n/inline-strings.test.ts).

Verification & Commands Executed

Real command outputs executed locally:

$ pnpm lint
> biome check .
Checked 91 files in 20ms. No fixes applied.

$ pnpm typecheck
> tsc --noEmit
(exit 0, 0 errors)

$ pnpm test
> vitest run
 Test Files  16 passed (16)
      Tests  186 passed (186)
   Start at  18:48:24
   Duration  728ms

$ pnpm build
> next build
▲ Next.js 16.3.8 (Turbopack)
✓ Running next.config.ts took 19ms
✓ Compiled successfully in 462ms
✓ Generating static pages using 14 workers (6/6) in 429ms
Route (app)
├ ○ /_not-found
├ ƒ /locks/[id]
├ ƒ /payee
...