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>

src-tauri/src/commands/repository.rs

@@ -52,6 +52,7 @@ pub struct RepositoryManagerWrapper(pub RepositoryManager);
 pub async fn repository_create(
     manager: State<'_, RepositoryManagerWrapper>,
     db: State<'_, crate::commands::storage::DatabaseWrapper>,
+    connectivity: State<'_, crate::commands::connectivity::ConnectivityMonitorWrapper>,
     server_url: String,
     user_id: String,
     access_token: String,
@@ -68,9 +69,18 @@ pub async fn repository_create(
     })?;
     debug!("[REPO] HTTP client created successfully");
 
-    // Create online repository
+    // Grab a connectivity reporter so the online repository's server outcomes
+    // drive the reachability state the UI observes (source of truth for the
+    // offline/online banner). See docs/architecture/07-connectivity.md.
+    let connectivity_reporter = {
+        let monitor = connectivity.0.lock().await;
+        monitor.reporter()
+    };
+
+    // Create online repository wired to connectivity reporting
     debug!("[REPO] Creating online repository...");
-    let online = OnlineRepository::new(Arc::new(http_client), server_url, user_id.clone(), access_token);
+    let online = OnlineRepository::new(Arc::new(http_client), server_url, user_id.clone(), access_token)
+        .with_connectivity(connectivity_reporter);
     debug!("[REPO] Online repository created");
 
     // Create offline repository with async-safe database service