feat(tui): keyboard-only ASCII redesign of config TUI

Remove all Button widgets — saves and plugin toggles are keyboard-only
(ctrl+s, e, d). Replace Header with a plain _TitleBar Static. Apply a
dark monochrome ASCII theme: +---+ borders on inputs, DataTable, and
tab panes; #0d0d0d background; grey/white palette. Disable mouse at the
driver level via run(mouse=False). Update save test to drive via ctrl+s.

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

tests/unit/test_config_tui.py

@@ -107,8 +107,8 @@ async def test_general_tab_save_persists_new_value(tmp_pyra_home):
         async with _TestApp().run_test() as pilot:
             widget = pilot.app.query_one(f"#{_fid('general.user_name')}", Input)
             widget.value = "Alice"
-            await pilot.pause()  # flush reactive update before click
-            await pilot.click("#save-general")
+            await pilot.pause()  # flush reactive update before key press
+            await pilot.press("ctrl+s")
 
     assert saved, "save_config was not called"
     assert saved[-1].general.user_name == "Alice"