feat(frame): read AUTO straighten from the photo, not the accelerometer

EXIF Orientation only knows 0/90/180/270, so a library still carries no
record of how the camera was held. The sensor had nothing to offer.

AUTO now measures the dominant line in the picture itself:
src/utils/horizon.ts runs a shear-projection search (coarse 1 deg over
-45..45, then a 0.5 deg refine) on a <=256px thumbnail, bails when no
line's score beats 3x the median, and returns the tilt in degrees.
App applies photoStraighten = -tilt, so preview and export share one
number exactly as the slider did.

Removes expo-sensors wiring, the horizonRoll state, effectiveStraighten
and the bubble-level overlay (autoRoll prop) from App/AdjustmentPanel/
Viewfinder. expo-sensors stays in package.json.

src/components/Viewfinder.tsx

@@ -33,6 +33,7 @@ import { TONE_SKSL, getToneUniforms, toneUniformArray } from '../utils/toneShade
 import { CINEMA_SKSL, getCinemaUniforms, cinemaOffFlat } from '../utils/cinemaShader';
 import { polaroidLayout, wallframeLayout, WALLFRAME_PNG_W, WALLFRAME_PNG_H } from '../utils/frameUtils';
 import { applyPhotoRotation } from '../utils/skiaImage';
+import { detectTilt } from '../utils/horizon';
 
 interface ViewfinderProps {
   mode: 'camera' | 'library';
@@ -54,9 +55,6 @@ interface ViewfinderProps {
   // on top of that turn.
   photoRotation?: 0 | 90 | 180 | 270;
   photoStraighten?: number;
-  // FRAME-tab AUTO: the device's lean in degrees, null while AUTO is off. Only
-  // drives the level line — the photo arrives already counter-rotated.
-  autoRoll?: number | null;
   // WATERMARK tab: double-tap ON the mark re-opens the text editor (App bumps
   // a nonce so the panel puts the caret back in its input).
   onWmEdit?: () => void;
@@ -164,6 +162,14 @@ export interface ViewfinderHandle {
    * fixed-ratio crop is active.
    */
   getCropRatioRect(): CropRect | null;
+  /**
+   * FRAME-tab AUTO straighten: tilt, in degrees, of the dominant near-
+   * horizontal line in the library photo — the horizon it was shot with.
+   * Null when the picture holds no line worth trusting, so App can leave the
+   * fine angle alone. Read from the quarter-turned source WITHOUT the current
+   * straighten, so pressing AUTO twice answers the same both times.
+   */
+  detectHorizon(): number | null;
 }
 
 const noiseEffect = Skia.RuntimeEffect.Make(`
@@ -237,7 +243,6 @@ const Viewfinder = forwardRef<ViewfinderHandle, ViewfinderProps>(function Viewfi
     wmEditing = false,
     photoRotation = 0,
     photoStraighten = 0,
-    autoRoll = null,
     wmRotation = 0,
     wallLandscape = false,
   }: ViewfinderProps,
@@ -338,6 +343,10 @@ const Viewfinder = forwardRef<ViewfinderHandle, ViewfinderProps>(function Viewfi
     () => applyPhotoRotation(loadedImage, photoRotation, photoStraighten),
     [loadedImage, photoRotation, photoStraighten]
   );
+  // Mirror of the two inputs of the above, for detectHorizon: the imperative
+  // handle is captured once (deps [ref]) and must read the LIVE source.
+  const horizonSrcRef = useRef({ image: loadedImage, quarter: photoRotation });
+  horizonSrcRef.current = { image: loadedImage, quarter: photoRotation };
   // Wall-frame PNG artwork (project root). Loaded lazily so other frames pay
   // no asset cost; source switches to '' when unselected (useImage → null).
   const wallImage = useImage(selectedFrame === 'wallframe' ? require('../../wallframe.png') : '');
@@ -580,6 +589,12 @@ const Viewfinder = forwardRef<ViewfinderHandle, ViewfinderProps>(function Viewfi
       getCropRatioRect() {
         return cropRatioRectRef.current;
       },
+      // AUTO straighten: the angle the picture asks for. Computed on demand
+      // (one <=256px thumbnail, tens of ms) — never on a slider drag.
+      detectHorizon() {
+        const src = horizonSrcRef.current;
+        return detectTilt(applyPhotoRotation(src.image, src.quarter, 0));
+      },
     }),
     [ref]
   );
@@ -2257,24 +2272,6 @@ const Viewfinder = forwardRef<ViewfinderHandle, ViewfinderProps>(function Viewfi
   const cinemaOn = cinemaEffect != null && !!cinemaParams;
   const useShaderPass = !!imageFitRect && ((toneEffect != null && toneOn) || cinemaOn);
 
-  // FRAME-tab AUTO: a bubble level that follows the device's own lean. The photo
-  // under it is already counter-rotated by the same angle, so this line is the
-  // only place the raw tilt is visible. Amber once within a degree of level.
-  const levelOverlay = autoRoll == null ? null : (
-    <View pointerEvents="none" className="absolute inset-0 items-center justify-center">
-      <View style={{ width: '55%', transform: [{ rotate: `${autoRoll}deg` }] }}>
-        <View style={{ height: 4, backgroundColor: 'rgba(0,0,0,0.5)' }} />
-        <View
-          style={{
-            height: 2,
-            marginTop: -3,
-            backgroundColor: Math.abs(autoRoll) <= 1 ? '#f59e0b' : '#ffffff',
-          }}
-        />
-      </View>
-    </View>
-  );
-
   // RETRO INSTANT (library): the whole picture is contain-fitted into the
   // card's photo window — nothing is masked or cropped, and the card is
   // centered on the black background like a physical print. The window keeps
@@ -2350,7 +2347,6 @@ const Viewfinder = forwardRef<ViewfinderHandle, ViewfinderProps>(function Viewfi
           onResponderMove={onLibTouchMove}
           onResponderRelease={onLibTouchEnd}
         />
-        {levelOverlay}
       </View>
     );
   }
@@ -2427,7 +2423,6 @@ const Viewfinder = forwardRef<ViewfinderHandle, ViewfinderProps>(function Viewfi
           onResponderMove={onLibTouchMove}
           onResponderRelease={onLibTouchEnd}
         />
-        {levelOverlay}
       </View>
     );
   }
@@ -2570,7 +2565,6 @@ const Viewfinder = forwardRef<ViewfinderHandle, ViewfinderProps>(function Viewfi
             onResponderMove={onLibTouchMove}
             onResponderRelease={onLibTouchEnd}
           />
-          {levelOverlay}
         </>
       ) : (
         <View className="items-center justify-center p-6">