Clear initial mypy typecheck baseline

[ZFordDev]

Mypy is now installed by CI and reports 15 type errors across 7 source files. Ruff is already clean; these are a separate type-safety baseline.

Groups:

• Optional Tkinter module typing in startup/selector.py.
• Dynamic logging handler metadata in logic/storage/sqlite_storage.py.
• Mixed settings value typing in ui/pyqt/settings_dialog.py.
• Optional Qt header and selection objects in ui/pyqt/task_list.py and calendar_view.py.
• Qt paintCell override annotations in calendar_view.py.
• Missing tkcalendar typing metadata in tkinter_ui.py.
• Optional startup selector result in startup/controller.py.

Acceptance criteria:

• python -m mypy src passes without globally disabling useful checks.
• Ruff lint and formatting remain green.
• Unit tests remain green.
• Narrow third-party or dynamic API exceptions are documented locally.