feat(agenda): split range picker into two lists with a concrete header

Group the range options into calendar-aligned (today / this week / this
month) and rolling (next 7 / 30 days / custom) lists. Add a header showing
the concrete span currently in effect — a single date for Today, the month
and year for This month, otherwise a start–end span — so it's clear what
the agenda is showing. The header appears only where a window is supplied
(the agenda pill), not in Settings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -294,6 +294,7 @@
     <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>
+    <string name="agenda_range_showing_label">Alle Termine für</string>
     <plurals name="agenda_range_days">
         <item quantity="one">%d Tag</item>
         <item quantity="other">%d Tage</item>