feat(settings): add 12/24-hour time format toggle

Add a TimeFormatPref (Auto/12h/24h) preference. Auto follows the device's
24-hour system setting; the others force a clock app-wide. The resolved
convention is provided once at the app root via LocalUse24HourFormat, so
every in-app time label reads it without per-screen plumbing.

A shared, pure TimeFormat helper (formatTimeOfDay / formatMinuteOfDay /
formatHourLabel / timeOfDayFormatter) is now the single source for all
time-of-day rendering. Routing every site through it also fixes a
pre-existing inconsistency: the week/day timeline gutters and the agenda
screen + widget hard-coded 24h, while event detail/edit/search/reminders
followed the locale — so a 12h-locale user previously saw mixed formats.

Covered: week & day timelines (gutter + event blocks), agenda screen and
widget, event detail/edit, search, and reminder notifications.

Closes #6 (Codeberg)

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

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

@@ -271,6 +271,10 @@
     <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_time_format">Time format</string>
+    <string name="settings_time_format_auto">Automatic</string>
+    <string name="settings_time_format_12h">12-hour (2:00 PM)</string>
+    <string name="settings_time_format_24h">24-hour (14:00)</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>