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/gradle.properties

@@ -2,8 +2,12 @@ org.gradle.jvmargs=-Xmx4g -Dfile.encoding=UTF-8
 # Desktop targets the Java 21 runtime (Markdown renderer 0.44.0 is
 # Java-21 bytecode). AGP is JDK-21-compatible, so the Android build is
 # unaffected (its bytecode target stays JVM 17 via minSdk/jvmTarget).
-# Point this at your local JDK 21 if the path differs.
-org.gradle.java.home=/usr/lib/jvm/java-21-openjdk
+#
+# The JDK-21 home is machine-specific, so it is NOT hardcoded here (a
+# committed path would break every other checkout). Set it per machine via
+# one of:
+#   - ~/.gradle/gradle.properties  ->  org.gradle.java.home=/path/to/jdk21
+#   - or export JAVA_HOME=/path/to/jdk21 before running ./gradlew
 org.gradle.caching=true
 org.gradle.configuration-cache=true