feat(viewer,ui04): E04 interactions — pause reason set, click lock/drag cancel, manual scrub, keyboard semantics, destroy

- pauseReasons Set (hover/focus/pointer/offscreen/hidden/manual + userPaused via Space, no auto-resume); 500ms transient resume with re-evaluation
- pointerdown target lock; >8px drag suppresses click and scrubs via nudgeOffset (window-clamped); removed photos clear pending target
- keyboard: Space pause toggle, Escape collapse, ArrowUp/Down scrub; input fields untouched
- aria-live status announcements (zh/en); viewport touch-action none
- e2e: hover freeze/resume, popup deep link hit, drag cancel, keyboard, slot disable destroy

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

apps/viewer/src/i18n/locales/en.ts

@@ -36,6 +36,8 @@ export default {
     // UI-04 v2 vertical photo stream window
     streamLabel: 'Event photo window',
     collapseWindow: 'Collapse photo window',
+    streamPaused: 'Photo scrolling paused',
+    streamResumed: 'Photo scrolling resumed',
   },
   download: {
     action: 'Download photo',