fix curses.newwin() crash on narrow terminals in TUI dialogs

show_confirm() sized its window purely from message content, with no
upper bound — a long line (the Cryptgeon password URL in the
post-issuance "send email?" prompt) could easily exceed a narrow
terminal's width (e.g. a serial console), making curses.newwin()
raise "curses function returned NULL" and crash the whole app.
show_cert_form() had the same unguarded newwin() call, just with a
fixed 13x62 size instead of content-driven.

Clamp both windows' height/width to the actual screen size, and wrap
newwin() itself in try/except as a last-resort fallback (declined
confirm / cancelled form) in case clamping still isn't enough.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_dialogs.py

@@ -62,6 +62,31 @@ def test_show_confirm_esc():
     assert result is False
 
 
+def test_show_confirm_clamps_width_to_screen_for_long_line():
+    """A long message line (e.g. a Cryptgeon password URL) must not make the
+    requested newwin() width exceed the screen — that's what made
+    curses.newwin() raise 'curses function returned NULL' on a narrow
+    terminal such as a serial console."""
+    stdscr = _make_stdscr(rows=24, cols=80)
+    win = _make_win()
+    win.getch.side_effect = [ord("y")]
+    long_url = "https://cryptgeon.example.com/note/" + "a" * 100 + "#" + "b" * 64
+    with patch("curses.newwin", return_value=win) as mock_newwin:
+        show_confirm(stdscr, f"Send email?\n\nPassword URL: {long_url}")
+    h, w, y, x = mock_newwin.call_args.args
+    assert w <= 80
+    assert x >= 0
+
+
+def test_show_confirm_newwin_failure_returns_false():
+    """If curses.newwin() still fails despite clamping, show_confirm must
+    degrade to 'declined' rather than crash the whole app."""
+    stdscr = _make_stdscr()
+    with patch("curses.newwin", side_effect=_curses.error("returned NULL")):
+        result = show_confirm(stdscr, "Do you want to continue?")
+    assert result is False
+
+
 # ---------------------------------------------------------------------------
 # show_cert_form tests
 # ---------------------------------------------------------------------------
@@ -102,3 +127,28 @@ def test_show_cert_form_cancel_button():
         result = show_cert_form(stdscr, cn="alice", email="alice@example.com")
     assert isinstance(result, CertFormResult)
     assert result.confirmed is False
+
+
+def test_show_cert_form_clamps_to_narrow_screen():
+    """The form's fixed 13x62 size must not exceed a smaller-than-usual
+    screen, which would make curses.newwin() raise."""
+    stdscr = _make_stdscr(rows=10, cols=40)
+    win = _make_win()
+    win.getch.side_effect = [27]
+    with patch("curses.newwin", return_value=win) as mock_newwin:
+        show_cert_form(stdscr, cn="alice", email="alice@example.com")
+    h, w, y, x = mock_newwin.call_args.args
+    assert h <= 10
+    assert w <= 40
+    assert y >= 0
+    assert x >= 0
+
+
+def test_show_cert_form_newwin_failure_returns_cancelled():
+    """If curses.newwin() still fails despite clamping, show_cert_form must
+    degrade to an unconfirmed result rather than crash the whole app."""
+    stdscr = _make_stdscr()
+    with patch("curses.newwin", side_effect=_curses.error("returned NULL")):
+        result = show_cert_form(stdscr, cn="alice", email="alice@example.com")
+    assert isinstance(result, CertFormResult)
+    assert result.confirmed is False