feat(settings): allow week to start on any day

Replace the AUTO/MONDAY/SUNDAY week-start enum with a sealed type
(Auto + Day(DayOfWeek)) so users can pick any of the seven days as the
first day of the week. The picker now lists "Automatic" plus all seven
localised weekday names; labels come from java.time display names rather
than per-day string resources.

Stored values round-trip by DayOfWeek.name, so the legacy MONDAY/SUNDAY
preferences migrate transparently. Garbage values fall back to Auto.

Closes #3 (Codeberg)

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

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

@@ -276,8 +276,6 @@
     <string name="settings_dynamic_color_unavailable">Erfordert Android 12 oder neuer</string>
     <string name="settings_week_start">Wochenstart</string>
     <string name="settings_week_start_auto">Automatisch</string>
-    <string name="settings_week_start_monday">Montag</string>
-    <string name="settings_week_start_sunday">Sonntag</string>
     <string name="settings_section_event_form">Termin-Formular</string>
     <string name="settings_form_fields_hint">Standardmäßig angezeigte Felder — alles Weitere liegt hinter \"Weitere Felder\"</string>
     <string name="settings_color_unsupported">Farben auf nicht unterstützten Kalendern erlauben</string>

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

@@ -269,8 +269,6 @@
     <string name="settings_dynamic_color_unavailable">Requires Android 12 or newer</string>
     <string name="settings_week_start">Week starts on</string>
     <string name="settings_week_start_auto">Automatic</string>
-    <string name="settings_week_start_monday">Monday</string>
-    <string name="settings_week_start_sunday">Sunday</string>
     <string name="settings_section_event_form">New event form</string>
     <string name="settings_form_fields_hint">Fields shown by default — everything else sits behind \"More fields\"</string>
     <string name="settings_color_unsupported">Allow colors on unsupported calendars</string>