fix: replace Ctrl-S confirm with Ctrl-G in Certificate Details form

Ctrl-S (0x13) is the XOFF flow-control character; terminals intercept it
before curses sees it, causing output suspension in Konsole and others.
Replaced with Ctrl-G (0x07 / BEL) which has no terminal-level meaning.
Updated hint bar and test stub accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

openvpncertupdate.py

@@ -617,7 +617,7 @@ def show_cert_form(
                 fields[name].draw()
 
         fields[active[focus]].place_cursor()
-        hint = "Tab=next  F5=regen password  Ctrl-S=confirm  Esc=cancel"
+        hint = "Tab=next  F5=regen password  Ctrl-G=confirm  Esc=cancel"
         try:
             win.addstr(h - 2, 2, hint[:w - 4], curses.color_pair(COLOR_DISABLED))
         except curses.error:
@@ -629,7 +629,7 @@ def show_cert_form(
         if key == 27:
             curses.curs_set(0)
             return CertFormResult(cn=cn, email="", password="", confirmed=False)
-        if key == 0x13:       # Ctrl-S: immediate submit
+        if key == 0x07:       # Ctrl-G: immediate submit
             curses.curs_set(0)
             return _submit()
         if key == 9:          # Tab