Add sync backend infrastructure and improve grid snapping

- Add haexSyncBackends table with CRDT support for multi-backend sync
- Implement useSyncBackendsStore for managing sync server configurations
- Fix desktop icon grid snapping for all icon sizes (small to extra-large)
- Add Supabase client dependency for future sync implementation
- Generate database migration for sync_backends table

src-tauri/database/migrations/meta/_journal.json

@@ -22,6 +22,13 @@
       "when": 1762263814375,
       "tag": "0002_loose_quasimodo",
       "breakpoints": true
+    },
+    {
+      "idx": 3,
+      "version": "6",
+      "when": 1762300795436,
+      "tag": "0003_luxuriant_deathstrike",
+      "breakpoints": true
     }
   ]
 }