feat: add Clear All button to CommsTab — purge all messages from DB

Adds trash icon button in Messages header that clears all communications
for the active group: DMs, channel messages, activity feed, seen tracking,
and dead letters. Shows Tauri warning dialog with confirmation before delete.

- Rust: clear_all_communications() in btmsg.rs (4 DELETE queries)
- Tauri command: btmsg_clear_all_comms
- Bridge: clearAllComms(groupId)
- UI: trash button with hover-red styling, resets all local state after clear

src-tauri/src/lib.rs

@@ -277,6 +277,7 @@ pub fn run() {
             commands::btmsg::btmsg_get_agent_heartbeats,
             commands::btmsg::btmsg_get_dead_letters,
             commands::btmsg::btmsg_clear_dead_letters,
+            commands::btmsg::btmsg_clear_all_comms,
             commands::btmsg::btmsg_queue_dead_letter,
             // Audit log
             commands::btmsg::audit_log_event,