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.h

@@ -37,8 +37,24 @@
 #ifndef TOOLS_EDITOR_H
 #define TOOLS_EDITOR_H
 
+#include "ui/win_types.h"
 #include <glib.h>
+typedef struct
+{
+    pid_t pid;             // Editor process PID
+    gboolean active;       // Thread running
+    gchar* result;         // Edited message
+    GError* error;         // Error if any
+    pthread_mutex_t mutex; // Protect access
+    pthread_t thread;      // Editor thread
+    gchar* filename;       // Temporary file path
+    gchar* editor_cmd;     // Editor command with filename
+} EditorTask;
+
+extern EditorTask editor_task;
 
 gboolean get_message_from_editor(gchar* message, gchar** returned_message);
+gboolean get_message_from_editor_async(gchar* message);
+void editor_process(ProfWin* window);
 
 #endif