chore(web): add date-fns dependency for activity feed (task-32)

- Add date-fns@^4.1.0 for relative time formatting
- Used by ActivityFeed component to display timestamps (e.g., "2 minutes ago")
- Required dependency for task-32 activity feed feature

Author: Mearman

apps/web/package.json

@@ -50,6 +50,7 @@
     "@vanilla-extract/recipes": "0.5.7",
     "@vanilla-extract/sprinkles": "1.6.5",
     "@xyflow/react": "12.10.0",
+    "date-fns": "4.1.0",
     "dexie": "4.2.1",
     "posthog-js": "1.318.1",
     "qrcode": "1.5.4",