macos: add participants overflow count badge in toolbar

[shindgew]

Implements #94.

Adds a +N overflow badge to the participants avatar stack in the chat toolbar for threads that have more than 3 participants.

Changes

• ParticipantAvatarStack now renders only the first 3 avatars + a circular accent badge showing the overflow count (using compact notation).
• Updated ParticipantsButton width calculation to reserve space for the badge.
• The stack is now public and handles its own truncation + badge logic.

Details

• Matches the existing visual language of avatar stacks (overlap, shadows, borders) used elsewhere in the app (e.g. reactions).
• The original overflowCount placeholder in the file is now used.
• Clean, scoped change: only two files, no behavior changes for ≤3 participants.

Testing

• Local verification on macOS for participant counts 0–3 (no badge) and 4+ (badge appears, width adjusts correctly).
• No impact on 1:1 chats (the button is only shown for threads).

Risks / Readiness

• Purely presentational / layout tweak in the toolbar.
• No security, performance, or data changes.
• Production ready (low risk, easy to iterate on badge styling later if needed — e.g. font size for very large numbers).