Fix spending-limits usage & persistence, wire shell i18n, run e2e in CI

[synkijojo-byte]

Closes #648, closes #649, closes #650, closes #651.

#648 — Compute spending-limits todayUsage from real activity

• src/app/api/spending-limits/route.ts imported getBackendApiBaseUrl from @/lib/api/config, but that helper did not exist — the production proxy threw at runtime (tests passed only because they mock the whole module). Added getBackendApiBaseUrl() reading the server-only MUX_BACKEND_URL, normalised, returning "" when unset so the route responds 503 instead of fabricating data. Registered MUX_BACKEND_URL in src/lib/env.ts and .env.example.
• src/app/api/demo/spending-limits/route.ts no longer returns a hardcoded todayUsage: 750. It derives the figure from the mock transaction store via the new computeTodayUsage() (src/lib/spending-limits/todayUsage.ts) — the sum of completed transaction amounts on the latest day. The real route already gets todayUsage straight from mux-backend (backend work is out of scope here).

#649 — Stop writing spending limits to localStorage as account state

• SpendingLimitsCard no longer reads or writes the spending-limits localStorage key. Limits are account state owned by the backend; a per-browser copy is wrong for a second device or a shared machine.
• On a failed load it now shows an explicit error plus clearly-labelled default limits (no silent cache fallback). todayUsage starts at 0 rather than a fabricated 750 placeholder.

#650 — Consume shellLabels in Sidebar and TopNav

• Sidebar nav items and TopNav (network-switcher labels, page-title map, breadcrumb "Home", sign-out label) now source copy from src/lib/i18n/shellLabels.ts. Added nav.users and header.breadcrumbHome; header.logout set to the existing visible text ("Sign out") so no user-facing copy changes.
• Repaired the pre-existing broken Sidebar.test.tsx (missing useRouter mock and imports) so the suite runs again.

#651 — Run Playwright e2e in CI

• Added an e2e-tests job to .github/workflows/ci.yml that installs Chromium and runs pnpm run test:e2e against the in-repo mock API routes (NEXT_PUBLIC_API_URL="").
• tests/ci-workflow.test.ts guards the job so it cannot silently regress.

Docs

README env table + smoke-tests section, docs/frontend-env-vars.md, and src/docs/API_Hooks.md updated to match.

Testing

• pnpm exec vitest run over every touched file: 92 passed.
• Compared full failing-test sets on this branch vs. clean staging for the touched directories: 0 new failures, 14 pre-existing failures fixed (the whole Sidebar.test.tsx suite + one obsolete cache test).
• Pre-existing, unrelated redness on staging is left untouched: parse errors in APIKeyModal.tsx / ApiKeysTable.tsx (already noted in playwright.config.ts) that block the typecheck job, and route tests failing on a missing isMockFallbackAllowed import in src/app/api/auth/login/route.ts.

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