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/common.h

@@ -171,6 +171,8 @@ typedef enum {
     RESOURCE_XA
 } resource_presence_t;
 
+extern gboolean background_mode;
+
 gboolean string_to_verbosity(const char* cmd, int* verbosity, gchar** err_msg);
 
 gboolean create_dir(const char* name);