feat: add Continue and Cancel buttons to Certificate Details form

Tab (or Down/Enter on a field) now cycles through the three text fields
and then onto [ Continue ] and [ Cancel ] buttons at the bottom of the
form.  Enter or Space activates the focused button.  Ctrl-G still
confirms immediately from any position.  Escape still cancels.

The form window is trimmed from h=16 to h=13 since the buttons replace
the now-redundant empty space below the password field.

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

tests/test_dialogs.py

@@ -78,16 +78,11 @@ def test_show_cert_form_cancel():
 
 
 def test_show_cert_form_confirm():
-    """Pressing Enter three times advances through all 3 fields and confirms.
-
-    The form has 3 active fields (cn, email, password).  Enter on field 0
-    advances to field 1; Enter on field 1 advances to field 2; Enter on
-    field 2 (the last) confirms.  We therefore need 3 Enter keypresses.
-    """
+    """Enter advances through the 3 fields to the Continue button; Enter on
+    Continue confirms.  That's 4 Enter keypresses total."""
     stdscr = _make_stdscr()
     win = _make_win(rows=20, cols=70)
-    # Three Enter keypresses to advance through each field and confirm.
-    win.getch.side_effect = [10, 10, 10]
+    win.getch.side_effect = [10, 10, 10, 10]
     with patch("curses.newwin", return_value=win):
         result = show_cert_form(stdscr, cn="bob", email="bob@example.com")
     assert isinstance(result, CertFormResult)
@@ -95,3 +90,15 @@ def test_show_cert_form_confirm():
     assert result.cn == "bob"
     assert result.email == "bob@example.com"
     assert len(result.password) > 0
+
+
+def test_show_cert_form_cancel_button():
+    """Tab to the Cancel button (4 Tabs from start) and press Enter cancels."""
+    stdscr = _make_stdscr()
+    win = _make_win(rows=20, cols=70)
+    # Tab×4: cn→email→password→Continue→Cancel, then Enter
+    win.getch.side_effect = [9, 9, 9, 9, 10]
+    with patch("curses.newwin", return_value=win):
+        result = show_cert_form(stdscr, cn="alice", email="alice@example.com")
+    assert isinstance(result, CertFormResult)
+    assert result.confirmed is False