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.

tsconfig.json

@@ -1,8 +1,8 @@
 {
   "compilerOptions": {
     "sourceMap": true,
-    "jsx": "react",
-    "target": "ES2016",
+    "jsx": "react-jsx",
+    "target": "ES2020",
     "module": "ES2020",
     "allowJs": true,
     "strict": true,
@@ -12,7 +12,7 @@
     "outDir": "dist",
     "rootDir": "./src",
     "skipLibCheck": true,
-    "lib": ["ES2016", "DOM"]
+    "lib": ["ES2020", "DOM"]
   },
   "exclude": ["node_modules", "dist"],
   "include": ["src"]