Add a tonal pill in the agenda's bottom-left corner showing the current
range; tapping it opens the existing AgendaRangePicker as a session-only
override. The override lives in AgendaViewModel (in-memory), so it survives
view switches and rotation but resets to the saved default when the app is
relaunched. The pill fills with the primary container while an override is
active, and a hint in the picker spells out the temporary nature.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>