feat(frontend): public channels, self-excluded member picker, editable display name

- create-channel modal: Public checkbox (default off), self excluded from
  member list (creator auto-added), shows member display names
- Settings modal to edit your own display name
- members panel shows names and marks (you)

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

src/auth/auth-context.ts

@@ -8,6 +8,7 @@ export type AuthContextValue = {
   logout: () => Promise<void>
   ensureFreshToken: () => Promise<string | null>
   refreshGuilds: () => Promise<void>
+  updateName: (name: string) => Promise<void>
 }
 
 export const AuthContext = createContext<AuthContextValue | null>(null)