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/0003_luxuriant_deathstrike.sql

@@ -0,0 +1,10 @@
+CREATE TABLE `haex_sync_backends` (
+	`id` text PRIMARY KEY NOT NULL,
+	`name` text NOT NULL,
+	`server_url` text NOT NULL,
+	`enabled` integer DEFAULT true NOT NULL,
+	`priority` integer DEFAULT 0 NOT NULL,
+	`created_at` text DEFAULT (CURRENT_TIMESTAMP),
+	`updated_at` integer,
+	`haex_timestamp` text
+);