No way to search or filter threads inside a forum channel

**Describe the bug**

A forum channel shows its threads as one list with no way to search or narrow it. There is no search box, no filter, and no sort control inside the forum view. Once a channel has more than a screenful of threads, finding an older one means scrolling.

Global search does index forum content: the search allowlist migration includes both `45001` and `45003`, and forum hits render through `SearchResultItem.tsx`. So the data is searchable, just not from the place where you are reading it.

Inside `desktop/src/features/forum/` there is no search handling at all. `ForumView.tsx` renders `useForumPostsQuery(channel)` straight into a list with no query or filter state. The only `filter` call in the feature is an array filter that builds the mention autocomplete list in `ForumThreadPanel.tsx`, unrelated to browsing threads.

**Steps to reproduce**

1. Open a forum channel with a number of threads
2. Look for a way to search titles, filter by author, or change the ordering
3. There is none, so scroll

**Expected behavior**

A search or filter affordance scoped to the current forum channel. Since the content is already indexed, filtering the visible thread list would cover most of it.

Related: #4585 asks for threads to be sorted by most recent reply rather than creation date, which is the same browsing problem from a different angle.

**Version and platform**

- Buzz version: 0.5.6
- OS: Windows 11 (10.0.26200)