feat: show calendar-week numbers in Month view (#25)

Add an opt-in left gutter to the Month grid showing the ISO calendar-week
number, gated by a new "Week numbers" display setting (default off). The
number is computed on each row's first day — the same basis as the Week
view's badge — so the two views agree, and rendered as a low-emphasis
onSurfaceVariant label so it recedes across all six rows rather than
competing with the event bars. The weekday header reserves a matching
gutter so the day columns stay aligned.

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

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

@@ -95,6 +95,14 @@ class SettingsPrefsTest {
         assertThat(prefs.showHourLines.first()).isTrue()
     }
 
+    @Test
+    fun `week numbers default off and round-trips`(@TempDir tempDir: Path) = runTest {
+        val prefs = SettingsPrefs(newDataStore(tempDir))
+        assertThat(prefs.showWeekNumbers.first()).isFalse()
+        prefs.setShowWeekNumbers(true)
+        assertThat(prefs.showWeekNumbers.first()).isTrue()
+    }
+
     @Test
     fun `autofocus event title defaults on and round-trips`(@TempDir tempDir: Path) = runTest {
         val prefs = SettingsPrefs(newDataStore(tempDir))