ComposeView: Replace mention dropdown with custom OutlinedCard list
Refactor the user mention suggestions UI to use a custom `OutlinedCard` containing a `LazyColumn` instead of the standard `DropdownMenu`. This provides a more tailored look and feel, including profile avatars in the search results.
- Replace `DropdownMenu` and `DropdownMenuItem` with `OutlinedCard`, `LazyColumn`, and custom `Row` items.
- Include user avatars in the mention results using `AsyncImage`.
- Switch `mentionDids` to `mutableStateMapOf` for better state tracking in Compose.
- Reduce the search typeahead limit from 8 to 5 in the `Bluesky` data layer.
- Cleanup: Remove large block of commented-out `OutlinedTextField` code.
- Dependency Update: Add `androidx.compose.foundation` to support the new UI components.