Add 2D focus positioning, fix rotation tracking, sync preview with effect

2D positioning:
- Add positionX parameter to BlurParameters (was only Y before)
- Update shader with uPositionX and uPositionY uniforms
- Single-finger drag now moves focus center anywhere on screen
- Update gradient mask generation for capture

Rotation tracking:
- Remove dampening from rotation gesture (1:1 tracking)
- Rotate gesture now directly tracks finger movement
- Preview effect rotates in sync with overlay

Overlay and shader sync:
- Both now use same positionX, positionY, angle parameters
- Preview blur effect matches overlay visualization

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/src/main/res/raw/tiltshift_fragment.glsl

@@ -10,7 +10,8 @@ uniform samplerExternalOES uTexture;
 
 // Effect parameters
 uniform float uAngle;           // Rotation angle in radians
-uniform float uPosition;        // Center position of focus (0-1)
+uniform float uPositionX;       // Horizontal center of focus (0-1)
+uniform float uPositionY;       // Vertical center of focus (0-1)
 uniform float uSize;            // Size of in-focus region (0-1)
 uniform float uBlurAmount;      // Maximum blur intensity (0-1)
 uniform vec2 uResolution;       // Texture resolution for proper sampling
@@ -19,15 +20,15 @@ varying vec2 vTexCoord;
 
 // Calculate signed distance from the focus line
 float focusDistance(vec2 uv) {
-    // Rotate coordinate system around center
-    vec2 center = vec2(0.5, uPosition);
-    vec2 rotated = uv - center;
+    // Center point of the focus region
+    vec2 center = vec2(uPositionX, uPositionY);
+    vec2 offset = uv - center;
 
     float cosA = cos(uAngle);
     float sinA = sin(uAngle);
 
-    // After rotation, measure vertical distance from center line
-    float rotatedY = -rotated.x * sinA + rotated.y * cosA;
+    // After rotation, measure perpendicular distance from center line
+    float rotatedY = -offset.x * sinA + offset.y * cosA;
 
     return abs(rotatedY);
 }