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>

.planning/ROADMAP.md

@@ -166,8 +166,10 @@ unblock a later item. Order is a plan, not a contract — revisit after each lan
 3. ~~Settings redesign & restructure~~ *(shipped v2.3.0 — grew into the full
    grouped-list blueprint across Settings + calendars + drawer; see "v2.3"
    above)*
-4. **Per-event color** *(next)* — reuses the calendar color picker/palette; closes the create/edit theme
-5. Duplicate event — detail action → prefilled create form; near-free on the tap-to-create prefill infra
+4. ~~Per-event color~~ *(shipped v2.4.0)* — palette calendars write
+   `EVENT_COLOR_KEY` (sync-safe); local/opted-in calendars write a raw
+   `EVENT_COLOR`; off-by-default setting for no-palette synced calendars
+5. **Duplicate event** *(next)* — detail action → prefilled create form; near-free on the tap-to-create prefill infra
 
 (Tier 2+ numbering below shifts accordingly; ranking unchanged.)