fix(connectivity): drive reachability from real repository traffic

The offline/online switch was janky because two independent systems decided
"online" and never communicated:

- ConnectivityMonitor owned is_server_reachable (drove the UI banner) but
  learned reachability only from a standalone /System/Info/Public ping loop
  and from auth/login calls.
- HybridRepository served all real data by racing cache-vs-server but never
  read or wrote reachability.

So the banner reflected a side-channel poller, not the system the user actually
experienced: a successful ping could read "online" while authenticated data
calls 401'd or timed out, and three different timeout regimes (5s ping / 30s
data / 100ms cache race) flapped against each other.

Unify into a single source of truth:

- Extract a cheap, cloneable ConnectivityReporter that owns all reachability
  transitions and event emission.
- OnlineRepository reports the outcome of every server request to the reporter,
  classified via RepoError: Ok/Authentication/NotFound/Server => reachable
  (the server answered), Network => offline candidate, Database/Offline =>
  ignored (not a server signal).
- Time-window debounce (OFFLINE_CONFIRM_WINDOW = 5s): flip offline only after
  sustained network failure; recover instantly on the first success.
- Demote the ping loop to an offline-only recovery probe (no online polling;
  real traffic is the signal when online).
- Frontend: navigator.onLine is now advisory (triggers a recheck instead of
  forcing offline); removed the dead markReachable/markUnreachable store methods.

Docs updated (README, 07-connectivity, 03-data-flow, 02-svelte-frontend) to
describe the new model and fix pre-existing drift (HTTP client is 30s timeout +
5s ping, not the documented 10s/base_url).

Tests: 12 connectivity tests (debounce, instant recovery, RepoError
classification through report_outcome). Full suite: 398 Rust + 384 frontend
passing, svelte-check clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/architecture/README.md

@@ -15,6 +15,7 @@ JellyTau uses a client-server architecture: business logic lives in a comprehens
 - **Events + Polling hybrid**: Rust emits events the frontend listens to, and the UI also polls status on short intervals in a few hot spots (e.g. queue status in `library/+layout.svelte`, playback progress in `VideoPlayer.svelte`).
 - **Handle-Based Resources**: UUID handles for stateful Rust objects.
 - **Cache-First**: Parallel queries with intelligent fallback.
+- **Single source of truth for reachability**: Server reachability is derived from the outcome of *real repository traffic*, not a side-channel poller. The `OnlineRepository` reports each server result to the `ConnectivityMonitor` (classified via `RepoError`), which applies a time-window debounce before declaring the server offline and recovers instantly on the first success. The standalone `/System/Info/Public` probe runs *only while offline*, as a recovery detector for idle sessions.
 - **Poison-tolerant locking**: Shared `std::sync` state is accessed via the `MutexSafe`/`RwLockSafe` helpers in `utils/lock.rs`, which recover a poisoned lock instead of cascading a panic across the player.
 - **Graceful backend init**: If a native player backend (MPV/ExoPlayer) fails to initialize, the app falls back to a no-op backend and emits a `backend-init-failed` event rather than crashing.
 
@@ -79,9 +80,12 @@ flowchart TB
         Core --> Storage
         Repository --> HttpClient
         Repository --> DatabaseService
+        Repository -->|"reports server outcome<br/>(success / RepoError)"| ConnectivityMonitor
     end
 ```
 
+> The `Repository --> ConnectivityMonitor` edge is the source of truth for the offline/online banner: every server request the user actually makes updates reachability. The monitor's own polling is now an offline-only recovery probe (see [07-connectivity.md](07-connectivity.md)).
+
 ---
 
 ## Detailed Documentation
@@ -187,7 +191,7 @@ src/lib/
 
 **What moved to Rust (~3,500 lines of business logic):**
 1. **HTTP Client** (338 lines) - Retry logic with exponential backoff
-2. **Connectivity Monitor** (301 lines) - Adaptive polling, event emission
+2. **Connectivity Monitor** (301 lines) - Reachability derived from real repository traffic, time-window debounce, offline-only recovery probe, event emission
 3. **Repository Pattern** (1061 lines) - Cache-first hybrid with parallel racing
 4. **Database Service** - Async wrapper preventing UI freezing
 5. **Playback Mode** (303 lines) - Local/remote transfer coordination