feat: upgrade toolchain and soften sync recovery after network blips

Bump Vite 8, matrix-js-sdk, and related deps; fix immer/vanilla-extract imports for new majors. Replace blocking sync error dialog with background recovery on wake/offline, and disable Vite auto-open in dev. Add feature handoff documentation index.

src/app/hooks/usePowerLevels.ts

@@ -1,6 +1,6 @@
 import { MatrixEvent, Room } from 'matrix-js-sdk';
 import { createContext, useCallback, useContext, useMemo, useState } from 'react';
-import produce from 'immer';
+import { produce } from 'immer';
 import { useStateEvent } from './useStateEvent';
 import { StateEvent } from '../../types/matrix/room';
 import { useStateEventCallback } from './useStateEventCallback';