fix(offline): initialize the CAWS backend reactively when connectivity returns

## Problem
(verified) `initialize()` runs `backend.initialize()` only if a one-shot connectivity check passes (`lib/blocs/app_bloc.dart:181-184`) — offline at launch means auth/services are never configured for the whole session, with no retry and no user feedback. `checkConnectivity()` is point-in-time only (`app_bloc.dart:198-206`).

## Proposed fix
Subscribe to `Connectivity().onConnectivityChanged` in the SystemInfoService from #589; on first regain, complete backend initialization and notify; surface an offline banner state for the router/login page instead of silently dead buttons (`login_page.dart:82` already checks connectivity per-tap, which can then simplify).

## Acceptance criteria
- [ ] Launch offline → go online → login works without app restart (manual + widget test with a mocked connectivity stream)
- [ ] Subscription cancelled on dispose (ties to the leaks EPIC)

**Effort:** M · **Priority:** P1 · **Change risk:** Low-medium