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/main/ipc-handlers.ts

@@ -115,4 +115,10 @@ export function registerIpcHandlers(): void {
   ipcMain.handle('redis:setTTL', async (_e, id: string, key: string, ttl: number) => {
     await redisService.setKeyTTL(id, key, ttl)
   })
+  ipcMain.handle('redis:selectDb', async (_e, id: string, db: number) => {
+    await redisService.selectDb(id, db)
+  })
+  ipcMain.handle('redis:dbSize', async (_e, id: string) => {
+    return redisService.dbSize(id)
+  })
 }