Gate Tor/relay startup on network reachability (#1389)

On a mesh-only/offline device the app used to bootstrap Tor and spin
Nostr relay reconnects forever ("connecting to Tor…"), wasting battery
even when there was provably no network path at all.

Add an NWPathMonitor-backed reachability signal (NetworkReachabilityMonitor)
and fold it into NetworkActivationService's activation gate:

- Tor bootstrap and relay connect/reconnect are now gated on the network
  path being usable. When the path is fully unsatisfied (no interface at
  all) we set autoStart off, shut Tor down, and disconnect relays instead
  of looping. When a usable path returns we resume.
- Conservative policy: only NWPath.Status.unsatisfied counts as offline.
  A flaky-but-present link stays "reachable" (Tor tolerates intermittent
  connectivity); we never tear down on the first hiccup.
- Transitions are debounced (ReachabilityDebounce, ~2.5s) so path flapping
  cannot thrash Tor/relay startup. The debounce is a pure value type,
  unit-tested without the Network framework or real timers.
- Starts optimistic (reachable) so nothing is suppressed before the first
  path evaluation arrives.
- BLE mesh never consults this gate and works fully offline.
- NWPathMonitor's background callback hops to the main actor before
  touching any state.

Surfaces NetworkActivationService.isNetworkReachable for UI to distinguish
"offline" from "connecting to Tor".

Tests: pure debounce (satisfied → allowed, unsatisfied → suppressed after
interval, flap debounced, recover-after-outage) plus service wiring
(unreachable suppresses Tor+relays, recovery resumes, loss disconnects).

Co-authored-by: jack <jackjackbits@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

bitchatTests/Services/NetworkActivationServiceTests.swift

@@ -111,6 +111,7 @@ final class NetworkActivationServiceTests: XCTestCase {
             mutualFavoritesPublisher: favoritesSubject.eraseToAnyPublisher(),
             permissionProvider: { permissionSubject.value },
             mutualFavoritesProvider: { favoritesSubject.value },
+            reachabilityMonitor: AlwaysReachableMonitor(),
             torController: torController,
             relayController: relayController,
             proxyController: proxyController,