fix(month): don't drag the continuous window back to 1900 (#38)

The continuous grid failed outright with "Could not read the calendar" the
moment it opened.

Before the first layout pass LazyListState reports no visible items, and the
screen defaulted that to item 0 — claiming January 1900 was on screen. The load
window widened around it, its pad ran past the epoch to index -4, and
yearMonthForIndex indexed Month.entries out of bounds; the flow's catch turned
the crash into the generic provider failure. Week indexing never showed this:
index 0 was a real week, so a bogus report only cost one wasted query.

- The visible-range report is skipped entirely while the list has no items,
  rather than standing in a default for them.
- clampMonthWindow holds any window inside 1900–2100, so a pad at either end
  can't produce an index with no month behind it.

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

app/src/test/java/de/jeanlucmakiola/calendula/ui/month/ContinuousMonthIndexTest.kt

@@ -88,6 +88,25 @@ class ContinuousMonthIndexTest {
         }
     }
 
+    @Test
+    fun `a window is clamped to the months that exist`() {
+        // The regression this guards: an empty visible-items list reported as
+        // "month 0", whose padded window ran to -4 — and yearMonthForIndex has
+        // no month for that, so the whole grid failed with "couldn't read".
+        assertThat(clampMonthWindow(-4..4)).isEqualTo(0..4)
+        val last = continuousMonthCount() - 1
+        assertThat(clampMonthWindow((last - 2)..(last + 4))).isEqualTo((last - 2)..last)
+        assertThat(clampMonthWindow(10..20)).isEqualTo(10..20)
+    }
+
+    @Test
+    fun `every index a clamped window can hold resolves to a month`() {
+        listOf(clampMonthWindow(-4..4), clampMonthWindow((continuousMonthCount() - 1)..(continuousMonthCount() + 3)))
+            .forEach { window ->
+                window.forEach { index -> yearMonthForIndex(index) }
+            }
+    }
+
     @Test
     fun `firstOfMonth is the 1st`() {
         assertThat(firstOfMonth(jun26)).isEqualTo(LocalDate(2026, 6, 1))