feat(phase3): key browser MVP

- useKeyStore (Pinia): scan, tree view, key data cache, db switching
- KeyBrowser: split pane with KeyList + KeyDetail
- KeyList: SCAN pagination, filter, tree toggle, load more
- KeyDetail: type badge, TTL display, delete, type-based editor dispatch
- StringEditor: view/edit with JSON auto-format
- HashEditor: field table, add/edit/delete fields
- StatusView: INFO dashboard with stat cards + section grid
- MainArea: tab host (Status / Keys)
- RedisService: selectDb, dbSize
- Preload: selectDb, dbSize IPC channels
- App.vue updated with MainArea layout

src/preload/index.d.ts

@@ -68,6 +68,8 @@ export interface ElectronAPI {
     hashSet: (id: string, key: string, field: string, value: string) => Promise<void>
     hashDel: (id: string, key: string, field: string) => Promise<void>
     setTTL: (id: string, key: string, ttl: number) => Promise<void>
+    selectDb: (id: string, db: number) => Promise<void>
+    dbSize: (id: string) => Promise<number>
   }
 }