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>

.env.example

@@ -2,3 +2,8 @@ VITE_CENTER_API_BASE=http://localhost:7001/api
 VITE_GUILD_API_BASE=http://localhost:7002/api
 VITE_GUILD_SOCKET_BASE=http://localhost:7002/realtime
 VITE_API_KEY=change-me-api-key
+
+# Brand overrides (build-time only; baked at build). Leave empty for Fabric defaults.
+VITE_APP_NAME=Fabric
+VITE_LOGO_URL=
+VITE_FAVICON_URL=