🐛 serialize Skia codec/bitmap close with in-flight readPixels

[guiyanakuang]

Closes #4437

Summary

• Fold produceCodecState + AnimatedFrames into a single LaunchedEffect(path) inside SkiaAnimatedImageView that owns the entire codec + bitmap lifecycle. Cleanup lives in one try { ... } finally { bitmaps.forEach { it.close() }; codec.close() }, which guarantees finally runs only after any in-flight withContext(ioDispatcher) { readPixels } returns naturally. No cross-composable handoff, no Mutex needed (and Mutex would not work anyway because onDispose cannot suspend).
• isPlaying is now read via rememberUpdatedState, so toggling play/pause no longer restarts the effect (previously safe only via a frameTick == 0 skip-prime trick; now structurally correct).
• The LoadingIndicator stays up until frame 0 is primed, eliminating the brief empty-bitmap flash on first frame.
• Public SkiaAnimatedImageView signature unchanged; preserved helpers (isAnimatedImage, frameDurationMs, MIN_FRAME_DURATION_MS) keep all 13 existing tests passing.

Test plan

• ./gradlew app:desktopTest --tests "com.crosspaste.ui.paste.side.preview.SkiaAnimatedImageViewTest" — all 13 tests pass
• ./gradlew :app:compileKotlinDesktop compiles clean
• Open the side preview on an animated GIF, then rapidly close it / scroll past — no SIGSEGV
• Toggle play/pause on a GIF — does not restart from frame 0 (would indicate the effect restarted and re-primed)