fix(mobile): full-screen media viewers + touch-pan for zoomed images (M3)

Mobile-audit batch 3. All changes mobile-gated (@media <=750px) so desktop is
unchanged.

- ModalWide: fill the phone screen (100vw/100vh, no radius) at <=750px instead
  of floating as an 85vw card. This also full-screens the file/PDF viewer and
  the avatar-crop editor on mobile (they share ModalWide) — intended.
- UserHero avatar viewer: new mobile-only ModalMobileFull class (no desktop
  effect) so it goes edge-to-edge on phones like the timeline lightbox.
- usePan: add touch support (single-finger drag, cleaned up on
  touchend/cancel/unmount) alongside the unchanged mouse path, so a zoomed image
  can be panned on a phone. Wired into ImageViewer and the MediaGallery lightbox.

Two review passes: mouse path byte-for-byte unchanged; desktop provably
unaffected; touch is gated to zoom!=1 so a non-zoomed image never hijacks
swipe/scroll.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/image-viewer/ImageViewer.tsx

@@ -19,7 +19,7 @@ export const ImageViewer = as<'div', ImageViewerProps>(
     const { t } = useTranslation();
     const saveFile = useSaveFile();
     const { zoom, zoomIn, zoomOut, setZoom } = useZoom(0.2);
-    const { pan, cursor, onMouseDown } = usePan(zoom !== 1);
+    const { pan, cursor, onMouseDown, onTouchStart } = usePan(zoom !== 1);
 
     const handleDownload = async () => {
       const fileContent = await downloadMedia(src);
@@ -100,6 +100,7 @@ export const ImageViewer = as<'div', ImageViewerProps>(
             src={src}
             alt={alt}
             onMouseDown={onMouseDown}
+            onTouchStart={onTouchStart}
           />
         </Box>
       </Box>