fix: resolve TypeScript 5.5 Uint8Array generic errors and missing vite/client types

TS 5.5 made typed arrays generic — Uint8Array now defaults to
Uint8Array<ArrayBufferLike>, which no longer satisfies Web Crypto's
BufferSource (requires ArrayBufferView<ArrayBuffer>).

- Add u8() helper that wraps .slice() results in new Uint8Array(),
  producing Uint8Array<ArrayBuffer> throughout crypto/index.ts
- Annotate all Uint8Array parameters/returns with <ArrayBuffer>
- Add 'vite/client' to tsconfig types so import.meta.env is recognized
- ws.ts: change socket.on() cleanup return to void (fixes useEffect type)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/tsconfig.json

@@ -2,6 +2,7 @@
   "compilerOptions": {
     "target": "ES2020",
     "lib": ["ES2020", "DOM", "DOM.Iterable"],
+    "types": ["vite/client"],
     "module": "ESNext",
     "moduleResolution": "bundler",
     "jsx": "react-jsx",