feat(agenda): bottom-left pill to override the range for the session

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>

app/src/main/res/values-de/strings.xml

@@ -293,6 +293,7 @@
     <string name="agenda_range_month">Nächste 30 Tage</string>
     <string name="agenda_range_custom">Benutzerdefiniert…</string>
     <string name="agenda_range_custom_hint">Tage</string>
+    <string name="agenda_range_override_hint">Nur vorübergehend — wird beim erneuten Öffnen von Calendula auf deinen gespeicherten Zeitraum zurückgesetzt.</string>
     <plurals name="agenda_range_days">
         <item quantity="one">%d Tag</item>
         <item quantity="other">%d Tage</item>