fix(editor): Ctrl-Z aborts editor and returns to profanity

Replaces the SIGTSTP=SIG_IGN inheritance approach (was commit 8) with a
detect-and-abort design that matches user expectation: Ctrl-Z inside
the editor brings you back to profanity instead of leaving the session
suspended.

- src/tools/editor.c: SIGTSTP reset back to SIG_DFL in the child so
  vim's :stop / Ctrl-Z actually stop the editor (revert previous
  SIG_IGN inheritance).
- src/tools/editor.c: new editor_check_stopped() — non-reaping waitpid
  poll for WIFSTOPPED. Lets prof_run detect Ctrl-Z / :stop / gdb-attach
  on the editor child.
- src/tools/editor.c: editor_emergency_kill() now sends SIGCONT before
  SIGTERM so the abort works on a STOPPED child too.
- src/tools/editor.c: _editor_exit_cb handles WIFSIGNALED cleanly with
  "Editor session cancelled." instead of printing garbage via
  WEXITSTATUS on a signaled child.
- src/profanity.c: SIGTSTP / SIGCONT signal handlers set flags consumed
  by the main loop. Editor STOPPED -> editor_emergency_kill (returns to
  the chat with the pre-alt+c readline buffer intact). Plain Ctrl-Z
  with no editor -> mutt-style drop the process group to the shell.
  SIGCONT -> refresh curses with clearok + ui_resize. While editor is
  active, swallow suspend_requested (race: vim's kill(0,SIGTSTP) beats
  waitpid seeing STOPPED — next poll catches it).
- src/tools/editor.h: declare editor_check_stopped and editor_is_active.

Verbose comment blocks across the branch tightened to one to two lines.

src/ui/core.c

@@ -64,10 +64,8 @@ static Display* display;
 
 static void _ui_draw_term_title(void);
 
-// Skip drawing while an external editor is active (ui_suspended) or after the
-// screen has been closed (endwin). The flag is authoritative and does not rely
-// on isendwin() staying TRUE, which a stray doupdate() during the session could
-// otherwise clear.
+// Fail-safe gate: authoritative flag (doesn't depend on isendwin staying
+// TRUE), plus the regular endwin check for shutdown.
 static gboolean
 _ui_redraw_suspended(void)
 {