feat(month): let the split style pull open to the full month (#53)

Drag the compact grid down and the day pane gives way to the month drawn
the paged style's way — real bars and pills, not dots; drag up to bring the
pane back. Split could show you which days were busy, or one day in full,
but never the month's events at once without a trip to Settings.

The month swipe grows a second axis to do it, so the two are locked to one
decision per gesture: independent horizontal and vertical detectors would
each see their own component of a diagonal drag and both fire. Paging keeps
ties, so an ambiguous drag still reads as it did.

A tap in the expanded grid picks the day and drops back, which makes it a
chooser you dip into rather than a mode you can get stranded in — so the
grid needs the selection outline too, which MonthGrid now takes optionally.

Expansion is state, not a preference: persisted, someone would expand it
once and later find their Split style changed with nothing to explain why.

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

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

@@ -385,6 +385,8 @@
     <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>
+    <string name="month_split_expand">Show the whole month</string>
+    <string name="month_split_collapse">Show the day\'s events</string>
     <string name="settings_quick_switch_header">Quick-switch button</string>
     <string name="settings_quick_switch_hint">Choose which views the top-right button cycles through, and drag to reorder them. Turned-off views stay reachable from the navigation menu.</string>
     <string name="settings_drawer_order_header">Navigation menu</string>