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/command/cmd_funcs.c

@@ -9527,16 +9527,7 @@ cmd_change_password(ProfWin* window, const char* const command, gchar** args)
 gboolean
 cmd_editor(ProfWin* window, const char* const command, gchar** args)
 {
-    auto_gchar gchar* message = NULL;
-
-    if (get_message_from_editor(NULL, &message)) {
-        return TRUE;
-    }
-
-    rl_insert_text(message);
-    ui_resize();
-    rl_point = rl_end;
-    rl_forced_update_display();
+    get_message_from_editor_async(NULL);
 
     return TRUE;
 }