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/AdjustmentPanel.tsx

@@ -89,8 +89,9 @@ interface AdjustmentPanelProps {
   photoStraighten: number;
   onPickRotation: (deg: 0 | 90 | 180 | 270) => void;
   onStraighten: (deg: number) => void;
-  autoHorizon: boolean;
-  onAutoHorizon: (on: boolean) => void;
+  // FRAME-tab AUTO: ask App to read the tilt of the photo itself and set the
+  // STRAIGHTEN angle that cancels it. A one-shot, not a mode.
+  onAutoStraighten: () => void;
   // FRAME tab (WALL FRAME only): hang the artwork landscape instead of the
   // default portrait rotation.
   wallLandscape: boolean;
@@ -122,9 +123,6 @@ interface AdjustmentPanelProps {
   // FRAME → WATERMARK sub-panel is open: App needs this to arm the mark's
   // drag/pinch layer on the image (the old `activeTab === 'watermark'`).
   onWmPanelChange: (open: boolean) => void;
-  // The ROTATE strip (or its STRAIGHTEN row) is open: App needs this to keep
-  // the image's bubble-level line on screen only while that strip is up.
-  onRotatePanelChange: (open: boolean) => void;
 }
 
 interface ChipDef {
@@ -225,8 +223,7 @@ export default function AdjustmentPanel({
   photoStraighten,
   onPickRotation,
   onStraighten,
-  autoHorizon,
-  onAutoHorizon,
+  onAutoStraighten,
   wallLandscape,
   onToggleWallLandscape,
   onToggleGeotag,
@@ -241,7 +238,6 @@ export default function AdjustmentPanel({
   wmRotation,
   onRotateWm,
   onWmPanelChange,
-  onRotatePanelChange,
 }: AdjustmentPanelProps) {
   // Double-tap on the mark in the viewfinder bumps wmEditNonce -> focus the
   // input again so the text can be retyped after the keyboard was dismissed.
@@ -277,12 +273,6 @@ export default function AdjustmentPanel({
   useEffect(() => {
     onWmPanelChange(openGroup === 'wm');
   }, [openGroup]);
-  // The bubble-level line on the image belongs to the ROTATE strip: it shows
-  // while that strip (or its STRAIGHTEN row) is open and leaves with the next
-  // chip, so it never lingers over the photo after the user moved on.
-  useEffect(() => {
-    onRotatePanelChange(openGroup === 'rotate' || openParam === 'straighten');
-  }, [openGroup, openParam]);
   // CROP is a plain-frame tool: the moment the selected frame drops it the chip
   // unmounts, so the ratio strip must close with it (no orphan CROP row).
   useEffect(() => {
@@ -593,12 +583,12 @@ export default function AdjustmentPanel({
       ],
       onPick: (v) => onUpdateCropRatio(v as CropRatio),
     },
-    // ROTATE: the quarter turns, the fine STRAIGHTEN slider and AUTO horizon in
-    // one strip. Its RESET undoes this parameter only (turn + angle + auto).
+    // ROTATE: the quarter turns, the fine STRAIGHTEN slider and the AUTO read
+    // in one strip. Its RESET undoes this parameter only (turn + angle).
     rotate: {
       label: 'ROTATE', off: '0',
       // 'straighten' while its slider row is open, so that chip highlights.
-      value: openParam === 'straighten' ? 'straighten' : autoHorizon ? 'auto' : String(photoRotation),
+      value: openParam === 'straighten' ? 'straighten' : String(photoRotation),
       options: [
         { v: 'reset', d: 'RESET' },
         { v: 'auto', d: 'AUTO' },
@@ -612,15 +602,13 @@ export default function AdjustmentPanel({
         if (v === 'reset') {
           onPickRotation(0);
           onStraighten(0);
-          onAutoHorizon(false);
         } else if (v === 'straighten') {
           toggleParam('straighten');
         } else if (v === 'auto') {
-          // AUTO is "snap to the sensor horizon": the manual angle is dropped
-          // first, so enabling it with a hand-set offset really levels the
-          // photo (re-tapping with it on still switches it back off).
-          if (!autoHorizon) onStraighten(0);
-          onAutoHorizon(!autoHorizon);
+          // AUTO reads the photo and REPLACES the fine angle with the one
+          // that levels its dominant line, so pressing it twice is the same
+          // as pressing it once.
+          onAutoStraighten();
         } else {
           onPickRotation(Number(v) as 0 | 90 | 180 | 270);
         }
@@ -671,19 +659,17 @@ export default function AdjustmentPanel({
     label: wbTempLabel(),
   });
 
-  // ROTATE's chip names every part of the angle it applies: the turn (dropped
-  // when it is 0 and AUTO is doing the work), the horizon mode, the fine angle.
+  // ROTATE's chip names every part of the angle it applies: the turn and the
+  // fine angle AUTO (or the slider) left on it.
   const rotateChip = (): ChipDef => {
     const parts = [
-      photoRotation !== 0 || !autoHorizon ? `ROTATE ${photoRotation}` : 'ROTATE',
-      autoHorizon ? 'AUTO' : null,
+      `ROTATE ${photoRotation}`,
       photoStraighten ? `${photoStraighten > 0 ? '+' : ''}${photoStraighten}°` : null,
     ];
     return {
       ...groupChip('rotate'),
       label: parts.filter(Boolean).join(' · '),
-      amberValue:
-        openGroup !== 'rotate' && (photoRotation !== 0 || photoStraighten !== 0 || autoHorizon),
+      amberValue: openGroup !== 'rotate' && (photoRotation !== 0 || photoStraighten !== 0),
     };
   };