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/index.tsx

@@ -2,7 +2,7 @@
 import React from 'react';
 import { createRoot } from 'react-dom/client';
 import { enableMapSet } from 'immer';
-import '@fontsource/inter/variable.css';
+import '@fontsource-variable/inter';
 import 'folds/dist/style.css';
 import { configClass, varsClass } from 'folds';