fix(app): apply whole-review fixes (HIGH/MEDIUM/LOW) + dead code & stale comments

HIGH:
- ntfy listener: replace blocking exhausted() loop with SSE read + capped
  exponential-backoff reconnect; 60s read timeout
- GatewayClient.stop(): reset HTTP leg (http, httpCursor, sseFailures,
  usingLongPoll, lastAck)
- non-atomic shared state -> synchronized/@Volatile/AtomicLong/
  CopyOnWriteArrayList

MEDIUM:
- mediaId path-traversal guard (isValidMediaId) at network/app/fs boundaries
- loadFromCache: move ts=0 pending bubbles to end, keep stored order
- attachment placeholder tracked by identity, not filename
- optimistic ChannelStore updates on favorite/icon/automation/default
- secure-store caching (desktop map, android store)
- secret passed to keyring via stdin (macOS + Linux)
- SecureStore.clear() clears deviceId/syncCursor/fcmToken/ntfy*
- random ids for system messages; SSE EOF reconnect delay
- PowerShell $ escaping; dispose() cancels job before saving flows
- wire up "Forget pairing" in Settings
- move machine-specific org.gradle.java.home to user-level gradle.properties

LOW + dead code + stale comments:
- .aac->audio/aac; locale-fixed cost/size; 3-digit hex; hour+ latency
- Backdrop.DEFAULT defined once; notification id 24-bit; channel id cap
- remove dead FileSource, unused protocol/theme/media constants, empty
  onDispose, SDK_INT<O guard, hostFromUrl
- fix stale WS/SSE, M1/M5, and milestone KDoc comments

Verified: Kotlin desktop+android host tests, 100/100 Python gateway tests,
LSP clean, installed & running on device.

app/shared/src/commonMain/kotlin/iris/IrisApp.kt

@@ -32,9 +32,9 @@ import iris.util.PairLink
 /**
  * Root composable shared by the Android and Desktop shells.
  *
- * M1: routes between the Connect screen (unpaired / auth failed) and the
- * Chat screen (paired). Later milestones add the channel list, search,
- * settings, and media (docs/10-android-app.md).
+ * Routes between the Connect screen (unpaired / auth failed) and the main
+ * app (paired), which hosts the channel list, chat, search, settings, and
+ * media (docs/10-android-app.md).
  */
 @Composable
 fun IrisApp(