feat(calendars): "Add to phone calendar" intent + multi-day bars in month grid

- New IconButton in the detail screen opens the system event composer
  (Google Calendar / Samsung / iCloud) pre-filled with title, range,
  location, and description via Intent.ACTION_INSERT on CalendarContract.
  Falls back to the .ics share path if no calendar app is installed.
- Replaced the per-cell event-dot row with horizontal bars that span the
  cells a multi-day event covers. Bars are laid out by a greedy
  lowest-lane assignment so overlapping events stack rather than collide;
  cells removed their horizontal padding so adjacent bars merge into one
  uninterrupted line.
- Bars round their ends only at the event's actual start/end (or at week
  boundaries), so a 3-day conference reads as one continuous pill across
  the row.
- Added MonthGridBarsTest (7 tests) covering single/multi-day, lane
  collision, longer-event tiebreak, and the empty/ghost edge cases.

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/icons/symbols/MaterialSymbols.kt

@@ -97,6 +97,7 @@ object MaterialSymbols {
     val EmojiEmotions = MaterialSymbol("\uEA22")
     val Error = MaterialSymbol("\uF8B6")
     val ErrorOutline = MaterialSymbol("\uF8B6")
+    val EventAvailable = MaterialSymbol("\uE614")
     val ExpandLess = MaterialSymbol("\uE5CE")
     val ExpandMore = MaterialSymbol("\uE5CF")
     val Explore = MaterialSymbol("\uE87A")