Phase 2 session stability: refresh queue, Bearer fallback, proactive refresh

Phase 1 auth security hotfixes: cookie-based session, bearer checks, router guard sync ...

- Remove access_token from OAuth callback URL (token leak fix)
- Add sanitizeReturnTo() to prevent open redirect on login/callback
- Add Bearer token expiration check in resolve_user_by_bearer()
- Add user status check in load_user_by_id() (block inactive/banned)
- Cache authStore.init() promise to prevent race condition
- Await auth init in router beforeEach guard
- Remove URL token parsing from main.js and LoginPage
- Remove hasTokenInUrl workaround from client.js 401 handler

Fix OAuth callback: extract access_token in main.js before auth init, prevent 401 redirect loop in client.js

Fix OAuth callback: remove refreshToken() from init() to prevent 401 cascade that clears access_token

Add server/dist build to repository for zero-dependency prod deploy