feat(tui): redesign chrome with unified key hints and help overlay

Introduce one key-hint grammar across every view: bold key, dim label,
priority-ordered hints that drop from the end instead of ellipsizing.
The month footer becomes a single split status bar with a friendly
selected date, the '?' mega-line becomes a real keyboard shortcut
panel, and the editor, agenda, search, picker, detail, and delete
frames share the same chips. Notifications carry check/warning marks.
Adds a live PTY smoke test covering view interaction and terminal
state restoration on q and Ctrl-C exits.

tests/editor_tests.cpp

@@ -47,6 +47,10 @@ void test_create_and_validation()
     check(editor.handle_key("\x13") == nocal::tui::EditorOutcome::active &&
               editor.focused_field() == nocal::tui::EditorField::title && !editor.error().empty(),
           "blank title blocks submission and focuses the invalid field");
+    check(editor.render(72, 18, true).find("\x1b[1;31m") != std::string::npos,
+          "validation errors render in the error style");
+    check(editor.render(72, 18, false).find("\x1b[") == std::string::npos,
+          "error frame stays plain with ANSI disabled");
 
     (void)editor.handle_key("Design review");
     focus(editor, nocal::tui::EditorField::end_time);
@@ -179,6 +183,8 @@ void test_rendering()
           "render fills requested height exactly");
     check(plain.find("NEW APPOINTMENT") != std::string::npos &&
               plain.find("Ctrl-S save") != std::string::npos &&
+              plain.find("Tab next field") != std::string::npos &&
+              plain.find("Esc cancel") != std::string::npos &&
               plain.find("[ ] timed appointment") != std::string::npos,
           "render exposes form identity and controls");
     check(plain.find("\x1b[") == std::string::npos, "ANSI can be disabled");