feat: username reset, system-message UX, meetings polish, sidebar identity

- auth: forgot-password now has Email/Username tabs; new public
  /api/auth-helpers/reset-by-username resolves a username and hands off to the
  existing reset flow (real email or admin-notify fallback)
- messages: conversations expose isSystem; System notices are read-only (no
  composer, no call button) and styled distinctly (shield icon + badge)
- meetings: compact, larger control bar; self tile shows real initials and an
  avatar (not black) when the camera is off; delete-room UI with confirm
- sidebar: username-only accounts show @username instead of a synthetic email

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/types/messaging.ts

@@ -50,6 +50,7 @@ export type ConversationSummary = {
   id: string;
   name: string; // display name: group name, or the other participant for a DM
   isGroup: boolean;
+  isSystem: boolean; // a one-way System notice (no replies / calls)
   participants: Participant[];
   lastMessage: ConversationMessage | null;
   unread: boolean;