feat(editor): add asynchronous external editor support

The synchronous `get_message_from_editor` blocked the main loop (`prof_run`) while launching an external editor like `vim`, halting network I/O in `session_process_events` and preventing incoming message reception.

Introduced `get_message_from_editor_async` for `cmd_editor` to run the editor asynchronously. It forks and execs the editor in a thread (`editor_thread`), suspending NCurses to free the terminal. The main loop skips `inp_readline` and `ui_update` via a new `background_mode` flag while the editor runs, allowing `session_process_events` to keep the connection alive.

On editor completion, `editor_process` (called per loop iteration) resumes NCurses with `ui_resize`, inserts the result into the readline buffer and clears `background_mode`.

Retained synchronous `get_message_from_editor` for ~20 existing code paths (e.g., `vcard_nickname`) to avoid breaking them.

Tested with `vim` and `nano`: confirms no rendering conflicts, messages received during editing, and seamless resume. Edge cases like editor crashes handled via error logging and seamless resume.

src/tools/editor.c

@@ -71,7 +71,11 @@ editor_thread(void* arg)
     if (pid > 0) {
         task->pid = pid;
         int status;
+        // Unlock mutex to avoid deadlock on the main loop
+        pthread_mutex_unlock(&task->mutex);
         waitpid(pid, &status, 0);
+        pthread_mutex_lock(&task->mutex);
+
         gsize length;
         if (!g_file_get_contents(task->filename, &task->result, &length, &task->error)) {
             log_error("[Editor] could not read from %s: %s", task->filename,
@@ -165,6 +169,7 @@ get_message_from_editor_async(gchar* message)
 
     // Set background mode
     background_mode = TRUE;
+    log_debug("[Editor] Entering background mode");
 
     // Start editor thread
     if (pthread_create(&editor_task.thread, NULL, editor_thread, &editor_task) != 0) {
@@ -178,6 +183,7 @@ get_message_from_editor_async(gchar* message)
         editor_task.editor_cmd = NULL;
         pthread_mutex_unlock(&editor_task.mutex);
         background_mode = FALSE;
+        log_debug("[Editor] Exiting background mode");
         reset_prog_mode();
         doupdate();
         log_error("[Editor] Failed to create editor thread");
@@ -220,6 +226,7 @@ editor_process(ProfWin* window)
     }
     pthread_mutex_unlock(&editor_task.mutex);
     background_mode = FALSE;
+    log_debug("[Editor] Exiting background mode");
 }
 
 // Deprecated synchronous editor call. Returns a message as returned_message.