Fix concurrency, lifecycle, performance, and config issues from audit

Concurrency & bitmap lifecycle:
- Defer bitmap recycling by one cycle so Compose finishes drawing before
  native memory is freed (preview bitmaps, thumbnails)
- Make galleryPreviewSource @Volatile for cross-thread visibility
- Join preview job before recycling source bitmap in cancelGalleryPreview()
  to prevent use-after-free during CPU blur loop
- Add @Volatile to TiltShiftRenderer.currentTexCoords (UI/GL thread race)
- Fix error dismiss race with cancellable Job tracking

Lifecycle & resource management:
- Release GL resources via glSurfaceView.queueEvent (must run on GL thread)
- Pause GLSurfaceView when entering gallery preview mode
- Shut down captureExecutor in CameraManager.release() (thread leak)
- Use WeakReference for lifecycleOwnerRef to avoid Activity GC delay
- Fix thumbnail bitmap leak on coroutine cancellation (add to finally)
- Guarantee imageProxy.close() in finally block

Performance:
- Compute gradient mask at 1/4 resolution with bilinear upscale (~93%
  less per-pixel trig work, ~75% less mask memory)
- Precompute cos/sin on CPU, pass as uCosAngle/uSinAngle uniforms
  (eliminates per-fragment transcendental calls in GLSL)
- Unroll 9-tap Gaussian blur kernel (avoids integer-branched weight
  lookup that de-optimizes on mobile GPUs)
- Add 80ms debounce to preview recomputation during slider drags

Silent failure fixes:
- Check bitmap.compress() return value; report error on failure
- Log all loadBitmapFromUri null paths (stream, dimensions, decode)
- Surface preview computation errors and ActivityNotFoundException to user
- Return boolean from writeExifToUri, log at ERROR level
- Wrap gallery preview downscale in try-catch (OOM protection)

Config:
- Add ACCESS_MEDIA_LOCATION permission (GPS EXIF on Android 10+)
- Accept coarse-only location grant for geotags
- Remove dead adjustResize (no effect with edge-to-edge)
- Set windowBackground to black (eliminates white flash on cold start)
- Add values-night theme for dark mode
- Remove overly broad ProGuard keeps (CameraX/GMS ship consumer rules)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/proguard-rules.pro

@@ -1,10 +1,5 @@
 # Add project specific ProGuard rules here.
+# CameraX and GMS Location ship their own consumer ProGuard rules.
 
-# Keep CameraX classes
--keep class androidx.camera.** { *; }
-
-# Keep OpenGL shader-related code
+# Keep OpenGL shader-related code (accessed via reflection by GLSL pipeline)
 -keep class no.naiv.tiltshift.effect.** { *; }
-
-# Keep location provider
--keep class com.google.android.gms.location.** { *; }