Add real-time tilt-shift preview for gallery images

Gallery imports now show the effect live as you adjust parameters,
matching the camera preview experience. Uses a downscaled (1024px)
source bitmap for fast recomputation via collectLatest, which
cancels stale frames when params change mid-computation.

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

app/src/main/java/no/naiv/tiltshift/camera/ImageCaptureHandler.kt

@@ -327,6 +327,15 @@ class ImageCaptureHandler(
         return rotated
     }
 
+    /**
+     * Applies tilt-shift effect to a bitmap for real-time preview.
+     * Runs on [Dispatchers.IO]. The caller owns the returned bitmap.
+     */
+    suspend fun applyTiltShiftPreview(source: Bitmap, params: BlurParameters): Bitmap =
+        withContext(Dispatchers.IO) {
+            applyTiltShiftEffect(source, params)
+        }
+
     /**
      * Applies tilt-shift blur effect to a bitmap.
      * Supports both linear and radial modes.