fix(editor): follow live terminal size in external editor

The compose editor is spawned via fork+execvp and inherits profanity's
LINES/COLUMNS, which hold the size captured at startup and are never
refreshed on resize. A curses editor (nano, vim, ...) honors them over
the real window, so it renders at the launch-time size after a resize.

Drop LINES/COLUMNS from the child's environment before forking so its
curses falls back to ioctl(TIOCGWINSZ). Assembling the env in the parent
lets the child only reassign environ instead of calling unsetenv()
between fork and exec, keeping it clear of unsetenv()'s allocator work in
the multithreaded fork->exec window. profanity itself is unaffected.

src/tools/editor.c

@@ -143,8 +143,8 @@ launch_editor(gchar* initial_content, void (*callback)(gchar* content, void* dat
     g_source_unref(sigchld_warmup);
 
     // Build the editor's env without LINES/COLUMNS pre-fork, so the child only
-    // swaps environ (async-signal-safe) instead of calling unsetenv(). The
-    // editor's (n)curses then reads the live window via ioctl(TIOCGWINSZ).
+    // reassigns environ instead of calling unsetenv() between fork and exec.
+    // The editor's (n)curses then reads the live window via ioctl(TIOCGWINSZ).
     gsize env_len = 0;
     while (environ[env_len]) {
         env_len++;
@@ -191,7 +191,7 @@ launch_editor(gchar* initial_content, void (*callback)(gchar* content, void* dat
     editor_pid = pid;
     g_child_watch_add((GPid)pid, _editor_exit_cb, ctx);
     g_strfreev(editor_argv);
-    g_free(editor_env); // array only; strings are borrowed from environ
+    g_free(editor_env); // frees the parent's array only; the child keeps its own COW copy until execvp
     return FALSE;
 }