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/styles/Modal.css.ts

@@ -1,6 +1,27 @@
 import { style } from '@vanilla-extract/css';
 
+const mobileFullscreen = {
+  minWidth: '100vw',
+  minHeight: '100vh',
+  width: '100vw',
+  height: '100vh',
+  borderRadius: 0,
+} as const;
+
 export const ModalWide = style({
   minWidth: '85vw',
   minHeight: '90vh',
+  '@media': {
+    // Fill the phone screen instead of floating as an 85vw card with margins.
+    '(max-width: 750px)': mobileFullscreen,
+  },
+});
+
+// Mobile-only full-screen: no desktop effect (keeps the modal's normal size),
+// but fills the viewport on phones. For dialogs that should stay a small card on
+// desktop but go edge-to-edge on mobile (e.g. the avatar viewer).
+export const ModalMobileFull = style({
+  '@media': {
+    '(max-width: 750px)': mobileFullscreen,
+  },
 });