Preserve sessions during network reconnects

[0xSolarPunk]

Problem

After laptop wake, the service worker can serve Garcon's cached shell before /api is reachable. Auth bootstrap then treats the transport failure as unauthenticated, redirects an existing session to /login, and may clear a valid token if user validation fails transiently.

Closes #413.

Fix

• Retry auth status and user validation with bounded backoff and a 5-second per-request timeout.
• Preserve stored tokens on transport and retryable server failures; clear only on authoritative 401/403 responses.
• Model exhausted retries as an unavailable/reconnecting state instead of logged out.
• Suppress login redirects while auth is unavailable.
• Retry automatically on browser online, visibility restoration, and a 15-second visible-tab recovery interval.
• Coalesce concurrent recovery checks and expose a manual retry screen.
• Prevent stale bootstrap checks from overwriting successful login, registration, or logout mutations.
• Restrict online recovery checks to unavailable sessions so healthy app state is not remounted.
• Replace raw Failed to fetch login errors with the localized network message.

Testing

• bun run test — passed
• bun run check — passed
• bun run build — passed
• bun run --cwd integration-tests typecheck — passed
• Targeted Lightpanda auth reconnect E2E — passed
• Isolated bun run start --port 0 startup smoke — passed
• git diff --check — passed

Regression coverage includes status and user-validation transport failures, exhausted retries, retry recovery, token retention, 401/403 invalidation, concurrent-check coalescing, stale login/registration races, protected-route retention, manual browser recovery, healthy online events, registration recovery, and useful login transport errors.