refactor(month): make the continuous style a stack of month blocks (#38)

The continuous style streamed weeks with no boundaries at all, which left it
hard to tell where one month ended: the only marker was a "Jul 1" label on the
1st, and every boundary week mixed two months' days into one row.

It is now a vertical stack of self-contained month blocks. Each block shows only
its own days — the boundary week keeps its seven columns so nothing shifts
sideways, but the neighbour month's cells are blank rather than filled with
duplicates of days shown again a block later — under a sticky month header with
whitespace either side. Scrolling stays continuous; only the reading changes.

- The coordinate space moves from absolute week index to absolute month index
  (two LazyColumn items per month: header, then block). Unlike week indices, it
  doesn't depend on the week-start preference, so changing that reflows the rows
  inside a block without moving the block or losing the scroll position.
- The sliding data window now loads months rather than weeks, widened to whole
  grid weeks at both ends so a bar reaching into a block from a clipped-off day
  still renders.
- `clipWeekToMonth` is the pure seam: it drops the neighbour month's pills and
  counts and cuts spanning bars back to the month's own columns, keeping a flat
  cap on the cut side so a bar reads as continuing past the block.
- The top bar carries the year in this style — the block's own header names the
  month, so repeating it two lines up was pure duplication.

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

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

@@ -379,7 +379,7 @@
     <string name="month_style_paged">Pages</string>
     <string name="month_style_paged_summary">One month at a time. Swipe sideways to change month.</string>
     <string name="month_style_continuous">Continuous</string>
-    <string name="month_style_continuous_summary">Scroll up and down through the weeks. No month is cut off and no day appears twice.</string>
+    <string name="month_style_continuous_summary">Scroll up and down through the months. Each month keeps to itself under its own heading, with no days borrowed from its neighbours.</string>
     <string name="month_style_split">Split</string>
     <string name="month_style_split_summary">A compact grid with dots for events, and the day you tap listed underneath.</string>
     <string name="month_split_no_events">Nothing scheduled</string>