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/commands/btmsg.rs

@@ -124,6 +124,11 @@ pub fn btmsg_clear_dead_letters(group_id: String) -> Result<(), String> {
     btmsg::clear_dead_letters(&group_id)
 }
 
+#[tauri::command]
+pub fn btmsg_clear_all_comms(group_id: String) -> Result<(), String> {
+    btmsg::clear_all_communications(&group_id)
+}
+
 #[tauri::command]
 pub fn btmsg_queue_dead_letter(
     from_agent: String,