feat(frontend): terminal restyle + dm + grouping + brand override

- Restyle: ink 'blueprint' surfaces + IBM Plex Mono / Major Mono
  Display, hairline borders, restrained motion, 12px radii. Fabric
  purple accent kept. Layout unchanged.
- Brand override (build-time VITE_ only): VITE_APP_NAME / VITE_LOGO_URL
  / VITE_FAVICON_URL via src/lib/brand.ts (applyBrand in main.tsx);
  login wordmark/logo + title + favicon. .env.example documented.
- dm x-type: create modal participant becomes single-select (radio)
  for dm and forces private; member right-click menu gains 'Create
  new DM channel' (non-unique, fresh channel each time).
- Channels sidebar grouped by x-type with group headers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.tsx

@@ -4,6 +4,9 @@ import { BrowserRouter, HashRouter } from 'react-router-dom'
 import './index.css'
 import App from './App.tsx'
 import { AuthProvider } from './auth/AuthContext'
+import { applyBrand } from './lib/brand'
+
+applyBrand()
 
 // The packaged desktop app loads the bundled SPA over file://, where the
 // History API has no server to back path routing — use HashRouter there.