feat(core): A.6.1 — use OS connectivity signals to drive reconnect

Extends the A.6 supervisor with an OS-level connectivity hint so a
returning network triggers a redial immediately instead of waiting
out the current backoff slot (up to 60 s). The watchdog remains the
authoritative loss detector — the OS signal is advisory.

* `chanora_core::NetworkState` (Unknown / Online / Offline) is owned
  by `ChanoraSession` via a `tokio::sync::watch::Sender`.
  `set_network_state()` / `network_state()` are the public accessors.
* The supervisor's watch-phase `select!` gains a `network_rx`
  branch: Offline pre-charges watchdog misses (capped at
  `MAX_MISSES - 1`) so the next probe failure trips immediately;
  Online clears stale misses. This shrinks UI-banner latency on a
  Wi-Fi drop from ~15 s to ~5 s.
* The reconnect-loop's backoff sleep races against Online: a
  transition cuts the sleep short and resets the attempt counter so
  future losses start at the smallest backoff window again.
* `chanora_bridge` adds `BridgeNetworkState` (mirror enum) and a
  sync `set_network_state(state)` function. On platforms with no
  signal wired the supervisor stays at Unknown and falls back to
  pure watchdog/backoff — no behavioural regression vs A.6.
* Flutter adds `connectivity_plus ^6.1.0` and wires
  `_wireConnectivity()` in `main()`: seeds with `checkConnectivity()`
  then forwards every `onConnectivityChanged` to the bridge,
  mapping any non-`none` transport to Online.

Verified on Moto G Stylus 5G (Android 14): `svc wifi disable && svc
data disable` for ~40 s — reconnect banner appeared promptly
because the watchdog was pre-charged. After `svc wifi enable && svc
data enable` the supervisor woke from its 15 s backoff slot and
reconnected within seconds; the channel tree re-rendered without
user action.

apps/chanora_flutter/pubspec.yaml

@@ -39,6 +39,7 @@ dependencies:
   cupertino_icons: ^1.0.8
   flutter_rust_bridge: 2.12.0
   freezed_annotation: ^3.1.0
+  connectivity_plus: ^6.1.0
 
 dev_dependencies:
   flutter_test: