feat: auto-sync, version history, merge, org policy, tamper guard

New modules:
- version-history.js: IndexedDB snapshot storage with pruning/rollback
- merge.js: three-way field-level merge for sync conflict resolution
- org-policy.js: team/org policy enforcement, compliance checking,
  invite codes, required mappings that can't be disabled
- tamper-guard.js: admin password to protect disable/clear/export
  actions, org policy can prevent disabling

Auto background sync:
- Gist/URL sync via chrome.alarms (MV3-safe, survives SW restarts)
- Configurable interval (5/15/30 min), push on local changes
- performAutoSync() orchestrates pull-then-conditional-push

Multi-device dashboard:
- Device auto-registration with UUID + browser/platform detection
- Device list embedded in sync data for cross-device visibility
- getDeviceInfo(), setDeviceName(), getDevices(), removeDevice()

Manifest changes:
- Added "alarms" permission for background polling

Service worker:
- Alarm listeners for auto-sync and org policy polling (hourly)
- Tamper guard message handlers
- Alarms set up on install and startup

WIP: Options UI integration pending for all new features.

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

manifest.firefox.json

@@ -13,7 +13,8 @@
     "storage",
     "activeTab",
     "scripting",
-    "notifications"
+    "notifications",
+    "alarms"
   ],
   "host_permissions": [
     "https://claude.ai/*",