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/test/java/de/jeanlucmakiola/calendula/data/prefs/SettingsPrefsTest.kt

@@ -49,6 +49,22 @@ class SettingsPrefsTest {
         assertThat(prefs.weekStart.first()).isEqualTo(WeekStartPref.SUNDAY)
     }
 
+    @Test
+    fun `time format defaults to auto and round-trips`(@TempDir tempDir: Path) = runTest {
+        val prefs = SettingsPrefs(newDataStore(tempDir))
+        assertThat(prefs.timeFormat.first()).isEqualTo(TimeFormatPref.AUTO)
+        prefs.setTimeFormat(TimeFormatPref.TWELVE_HOUR)
+        assertThat(prefs.timeFormat.first()).isEqualTo(TimeFormatPref.TWELVE_HOUR)
+    }
+
+    @Test
+    fun `hour lines default off and round-trips`(@TempDir tempDir: Path) = runTest {
+        val prefs = SettingsPrefs(newDataStore(tempDir))
+        assertThat(prefs.showHourLines.first()).isFalse()
+        prefs.setShowHourLines(true)
+        assertThat(prefs.showHourLines.first()).isTrue()
+    }
+
     @Test
     fun `garbage stored enum falls back to default`(@TempDir tempDir: Path) = runTest {
         val store = newDataStore(tempDir)