plugin(collab-buckets): collaborative buckets via relay

End-to-end shared buckets between relay tenants. Bucket metadata + members
sync through new relay routes (collab-buckets, collab-cards, collab-locks);
card content rides the existing inter-tenant encryption hop. Edit locks,
ownership transfer, LWW conflict stashing, undo for local-bucket destructive
actions, "Make collaborative…" upgrade path, and a resilient outbox for
relay-down create attempts. See plugins/collab-buckets/README.md and
TODO.md for the full picture.

Core hooks added so the plugin can integrate without monkey-patching:
- buckets.js context menu: collab-aware rename/color/delete + Members &
  owner / Make collaborative entries
- editor.js dispatches hyprcrd:editor-open / hyprcrd:editor-close for the
  per-card edit-lock heartbeat
- app.js exposes window.state for the plugin's MutationObserver
- index.php POST/PATCH/DELETE on /api/buckets refuse to mutate collab
  buckets directly (must go through the relay)
- Database.php adds buckets.collab_uid column + unique partial index
- import handler now restores color and collab_uid (was silently dropping
  color, which caused the test runner's auto-restore to wipe custom
  bucket colors)
- relay bootstrap creates collab_buckets, collab_bucket_members,
  collab_cards, collab_card_locks tables and prunes tombstones / expired
  locks on every request

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/plugins/collab-buckets/plugin.json

@@ -0,0 +1,10 @@
+{
+    "name": "collab-buckets",
+    "version": "1.0.0",
+    "description": "Collaborative buckets shared between relay tenants. Polls the relay for bucket updates and shows pulsing header tiles with unread indicators.",
+    "frontend": {
+        "js":   ["collab-buckets.js"],
+        "css":  ["collab-buckets.css"]
+    },
+    "backend": "backend.php"
+}