release: cut v2.4.0 — per-event colors

Optional per-event color in the event form. The read/render path already
resolved EVENT_COLOR with a calendar fallback; this adds the write side and
the picker.

- Palette-backed calendars (Google, some CalDAV) pick from the account's
  Colors (TYPE_EVENT) and write EVENT_COLOR_KEY, so the color round-trips
  through sync; local calendars write a raw EVENT_COLOR from the shared
  CALENDAR_COLOR_PALETTE. Never writes a raw color to a palette calendar.
- Swatch row + palette extracted to ui/common/ColorSwatchRow.kt (shared with
  the calendar editor). Switching calendars resets the choice (keys are
  account-scoped); a "Reset" action returns to the calendar color.
- New "Allow colors on unsupported calendars" setting (off by default)
  extends the raw path to no-palette synced calendars, with an honest
  "may not survive sync" warning on the picker and in Settings.
- Color flows through insert / dirty-checked update / occurrence-exception;
  mapper, form, and repository tests added.

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

app/src/main/java/de/jeanlucmakiola/calendula/domain/Models.kt

@@ -58,8 +58,25 @@ data class EventDetail(
     val eventTimezone: String? = null,
     /** This device user's own response (`Events.SELF_ATTENDEE_STATUS`). */
     val selfStatus: AttendeeStatus = AttendeeStatus.Unknown,
+    /**
+     * The event's own raw colour (`Events.EVENT_COLOR`), null when the event
+     * inherits its calendar's colour. Unlike [EventInstance.color] (which
+     * already folds in the calendar fallback for display) this stays null so
+     * the edit form can tell "has own colour" from "inherits".
+     */
+    val eventColor: Int? = null,
+    /** The event's `Events.EVENT_COLOR_KEY` (a calendar-palette key), or null. */
+    val eventColorKey: String? = null,
 )
 
+/**
+ * One selectable event colour published by a calendar's account
+ * (`CalendarContract.Colors`, `TYPE_EVENT`): [key] is the account-scoped
+ * `COLOR_KEY` written as `EVENT_COLOR_KEY` (so the colour survives sync),
+ * [argb] is the swatch it renders as.
+ */
+data class EventColorOption(val key: String, val argb: Int)
+
 data class Attendee(
     val name: String,
     val email: String?,