feat: annotation UX overhaul, animated packing, toolbar redesign

- PinOverlay: zero-allocation rendering with in-place Graphics/Text updates,
  fix WebGL texture crash (addressModeU null) from orphaned Text objects
- Pins always visible on media, track during drag via updatePositions() fast path
- Replace eraser with Review mode in toolbar tool group (orange gradient)
- Add tool instruction hints bar below toolbar (context-sensitive per tool)
- Feedback panel: glassmorphism, improved ThreadDetail/CommentItem/ThreadListItem
- Timestamps: relative with "ago" suffix, full date on hover tooltip
- De-emphasize pin numbers, show status text in thread list
- Animated arrangement transitions (easeOutCubic, 320ms) for pack/grid/row/column
- Disable snap guides (user requested)
- Paste now selects newly created items
- Remove eraser keyboard shortcuts, remap tool shortcuts
- Orphaned thread cleanup: safe destroy check for deleted media pins

frontend/src/utils/relativeTime.ts

@@ -1,11 +1,25 @@
 export function relativeTime(iso: string): string {
-  const diff = Date.now() - new Date(iso).getTime();
+  const date = new Date(iso);
+  const diff = Date.now() - date.getTime();
   const mins = Math.floor(diff / 60000);
   if (mins < 1) return 'now';
-  if (mins < 60) return `${mins}m`;
+  if (mins < 60) return `${mins}m ago`;
   const hrs = Math.floor(mins / 60);
-  if (hrs < 24) return `${hrs}h`;
+  if (hrs < 24) return `${hrs}h ago`;
   const days = Math.floor(hrs / 24);
-  if (days < 7) return `${days}d`;
-  return new Date(iso).toLocaleDateString();
+  if (days < 7) return `${days}d ago`;
+  // Show date for older comments
+  return date.toLocaleDateString(undefined, { month: 'short', day: 'numeric' });
+}
+
+/** Full timestamp for tooltip hover */
+export function fullTimestamp(iso: string): string {
+  const date = new Date(iso);
+  return date.toLocaleString(undefined, {
+    month: 'short',
+    day: 'numeric',
+    year: 'numeric',
+    hour: '2-digit',
+    minute: '2-digit',
+  });
 }