camera: app-lifecycle pause / resume (Phase 6 polish)

Camera page kept the session running while the host app was
backgrounded — wastes battery, holds the hardware, and blocks
other apps from grabbing the camera. Add per-platform observers
that pause/resume the session on app foreground/background, with
a uniform `pauseForBackground` / `resumeForForeground` pair on the
shared CameraInstance.

Behaviour:
  - On background: any in-flight recording is hard-cancelled
    (matches every messaging app — the take ends with the app
    switch). The session stops so the OS can release the camera.
  - On foreground: session restarts iff it had been running.
    Emits `sessionInterrupted` (`reason: appBackgrounded`) and
    `sessionResumed` events so the Dart side can surface UX
    affordances if needed.

iOS — `ios/Classes/Camera/CameraInstance+iOS.swift`:
  Subscribes to UIApplication.{willResignActive, didBecomeActive}
  notifications. Work hops onto sessionQueue so AV mutations stay
  serialised. Storage uses the shared
  `CameraInstance.lifecycleCleanup` closure slot — extension
  doesn't need to add stored properties.

Android — added `androidx.lifecycle:lifecycle-process:2.7.0`,
  observes `ProcessLifecycleOwner.get().lifecycle`. ON_STOP →
  `pauseForBackground` (cancels recording + drops
  CustomLifecycleOwner to CREATED → CameraX releases camera).
  ON_START → `resumeForForeground`. Observer add/remove on main
  thread per `ProcessLifecycleOwner` contract.

macOS — `macos/Classes/Camera/CameraInstance+macOS.swift`:
  Intentional no-op. macOS desktop background semantics are
  softer; the chat composer's Card dialog typically stays
  foregrounded. Slot is wired so the shared
  `observeLifecycle()` call still compiles.

Verified: all four platforms (iOS / Android / macOS / app tests)
build clean. Pod install picks up the new iOS extension file
once Pods/ is fresh — `flutter clean` if mid-iteration.

android/build.gradle

@@ -63,6 +63,11 @@ dependencies {
     implementation "androidx.camera:camera-lifecycle:$cameraxVersion"
     implementation "androidx.camera:camera-view:$cameraxVersion"
     implementation "androidx.camera:camera-video:$cameraxVersion"
+    // ProcessLifecycleOwner so CameraInstance can release the camera
+    // when the host app backgrounds (and re-acquire on foreground).
+    // camera-lifecycle pulls in lifecycle-common but not the process
+    // observer; this adds it.
+    implementation 'androidx.lifecycle:lifecycle-process:2.7.0'
     // Pure-Kotlin/Java QR decoder. ~470 KB jar, no Play Services dep.
     implementation 'com.google.zxing:core:3.5.3'
 }