feat(timeline): optional hour separator lines in week & day view

Add a showHourLines preference (default off — the existing clean look).
When on, the week and day timelines draw a faint outline-variant line at
each hour boundary, sitting over the column background but beneath event
blocks. The toggle is provided app-wide via LocalShowHourLines and applied
through a reusable hourSeparatorLines() modifier.

Closes #5 (Codeberg)

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

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

@@ -275,6 +275,8 @@
     <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_hour_lines">Hour lines</string>
+    <string name="settings_hour_lines_summary">Show a separator line at each hour in week and day view</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>