feat(ios): add Home Screen and Lock Screen widgets for mentions and agent replies

## Summary

Add optional iOS Home Screen and Lock Screen widgets that surface the most useful Buzz activity at a glance, especially mentions and replies from agents.

## User need

For people who use Buzz primarily from an iPhone—particularly in a personal workspace with several specialized agents—opening the app and checking multiple channels is currently the only way to see whether something needs attention. A small widget would make Buzz much easier to use as an ambient personal assistant hub.

## Proposed widget set

Start with one compact, privacy-conscious widget:

- unread mention count;
- number of new agent replies;
- timestamp of the most recent update;
- tap to deep-link into Activity or the relevant channel.

Possible later variants:

- favorite-channel shortcuts;
- a medium Activity widget with recent items;
- agent availability/working status, if the presence data is reliable enough.

## Privacy and security

- Default to counts and generic labels, not message bodies or channel names.
- Let users explicitly enable previews.
- Redact sensitive content while the device is locked, following the user's notification-preview preference where possible.
- Keep widget data scoped to the current Buzz identity/community and clear it on logout or identity removal.

## Implementation notes

A WidgetKit extension could read a bounded, signed-in-state snapshot shared by the Buzz app through an App Group. The widget should not need to maintain its own relay connection. The parent app or push-processing path can refresh the snapshot and request timeline reloads within iOS limits.

## Suggested acceptance criteria

- [ ] Small Home Screen widget shows mentions and agent-reply counts.
- [ ] Tapping opens the correct Buzz destination.
- [ ] Lock Screen presentation exposes no message content by default.
- [ ] Stale/offline state is clearly indicated.
- [ ] Logging out or removing an identity clears shared widget data.
- [ ] Multiple identities/communities cannot leak data into one another.
- [ ] Widget updates are energy-efficient and tolerate iOS refresh throttling.

## Related search

No existing issue specifically requesting an iOS Home Screen or Lock Screen WidgetKit extension was found using `WidgetKit`, `iOS widget`, `home screen widget`, and `lock screen widget` searches.