feat(timetable): pinia store + CSS tokens (Session 4 steps 5+7)

useTimetableStore — pinia composition store carrying:
- activeDayId synced to ?day query param at the page level
- selectedPerformanceId for popover anchor + keyboard focus
- drag state (dragPerformanceId / dragOriginSnapshot / dragGhost) for
  optimistic preview + 409 rollback
- statusFilter (defaults: all on except cancelled, per prototype §4.7)
- searchQuery for the wachtrij filter

styles/tokens/_timetable.scss — RFC v0.2 D21:
- 9 status palettes (bg / border / fg / dot custom properties)
- cancelled-hatch repeating gradient
- conflict / capacity-warn / capacity-critical / B2B / trashed colours
- lane geometry (height, gap, padding, block radius)
- canvas + axis backgrounds and tick lines
- drag-ghost + focus-ring + day-tab chrome
- tt-cascade-pulse keyframe animation for D18 cascaded[] visualisation

Imported once via assets/styles/styles.scss so the variables are available
everywhere via var(--tt-…).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/app/src/assets/styles/styles.scss

@@ -1 +1,4 @@
 // Write your overrides
+
+// RFC-TIMETABLE v0.2 D21 — status palette + geometry custom properties.
+@use "@/styles/tokens/timetable";