feat: TUI widget primitives (colors, InputField, draw helpers)

Implement COLOR_* constants, initialization, box/centered drawing,
clamp utility, and InputField with handle_key/value logic for pure
text editing without live curses. All tests pass.

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

tests/test_widgets.py

@@ -0,0 +1,53 @@
+from unittest.mock import MagicMock
+import curses as _curses
+
+# Stub just enough of the curses API for InputField to import
+_curses.color_pair   = lambda x: 0
+_curses.A_UNDERLINE  = 0
+_curses.A_BOLD       = 0
+_curses.KEY_BACKSPACE = 263
+_curses.KEY_LEFT     = 260
+_curses.KEY_RIGHT    = 261
+_curses.KEY_HOME     = 262
+_curses.KEY_END      = 360
+_curses.KEY_DC       = 330
+
+from openvpncertupdate import InputField, clamp
+
+
+def _field(initial="", mask=False):
+    win = MagicMock()
+    win.getmaxyx.return_value = (1, 80)
+    return InputField(win, 0, 0, 40, initial=initial, mask=mask)
+
+
+def test_initial_value():
+    assert _field("hello").value == "hello"
+
+def test_backspace_removes_last():
+    f = _field("hello")
+    f.handle_key(_curses.KEY_BACKSPACE)
+    assert f.value == "hell"
+
+def test_type_appends():
+    f = _field("hi")
+    f.handle_key(ord("!"))
+    assert f.value == "hi!"
+
+def test_left_then_insert():
+    f = _field("ac")
+    f.handle_key(_curses.KEY_LEFT)
+    f.handle_key(_curses.KEY_LEFT)
+    f.handle_key(ord("b"))
+    assert f.value == "bac"
+
+def test_delete_key():
+    f = _field("abc")
+    f.handle_key(_curses.KEY_HOME)
+    f.handle_key(_curses.KEY_DC)
+    assert f.value == "bc"
+
+def test_clamp_bounds():
+    assert clamp(5, 0, 10) == 5
+    assert clamp(-1, 0, 10) == 0
+    assert clamp(11, 0, 10) == 10