Pinned messages, search, and history pagination

## Summary
Implement pinned messages, message search, and history pagination with functional Discord-like parity for web desktop first.

## Scope
- Pin and unpin messages with appropriate permissions and first-class pinned-message browsing.
- Search message history with useful filters and navigation back to result context.
- Paginate or lazy-load long histories without breaking unread state or jump behavior.
- Preserve direct links and context jumps into older history.

## Acceptance Criteria
- [ ] Users with permission can pin and unpin messages and browse the pinned list for a channel or thread.
- [ ] Users can search history and navigate from results back into channel context.
- [ ] Older history loads incrementally without breaking ordering, unread state, or scroll position.
- [ ] Search and pin behavior respect permission and visibility boundaries.

## Operational Notes
- Search quality should prioritize functional user-visible parity first; ranking sophistication can improve later.
- Large self-hosted channels need predictable query and pagination performance.