Add Backup Management GUI (Phase 3-4)

Web GUI Implementation:
- Backup.vue: Storage dashboard + Snapshot management + Scheduler config
- Router: Add /backup route
- Home.vue: Add Backup management card
- Tauri commands: 10 backup API endpoints

Features:
- Storage stats (total/used/free, dedup/compression ratios)
- Snapshot list with create/delete/restore actions
- Backup scheduler configuration (enabled, interval, max_snapshots)
- Run backup now button
- Send/Receive placeholders

Tauri Commands:
- get_storage_stats, list_snapshots
- create_snapshot, delete_snapshot, restore_snapshot
- get_backup_stats, get_backup_config, set_backup_config
- run_backup

Build: cargo build (Tauri) ✅ 5 warnings
Tests: 495 markbase-core + 201 smb-server = 696 total

markbase-tauri/src-tauri/src/commands/mod.rs

@@ -5,6 +5,7 @@ pub mod diagnostic;
 pub mod management;
 pub mod health;
 pub mod monitor;
+pub mod backup;
 
 pub use file_ops::*;
 pub use install::*;
@@ -12,4 +13,5 @@ pub use config::*;
 pub use diagnostic::*;
 pub use management::*;
 pub use health::*;
-pub use monitor::*;
+pub use monitor::*;
+pub use backup::*;