feat(ui): consolidate shared UI patterns into reusable components

- Token cleanup: replace raw hex colors (#c084fc, #60a5fa, #6ee7b7, #fdba74)
  in BoardCard.vue with nexus-tokens.css CSS variables (--clr-iris, --clr-bao,
  --clr-agent, --clr-review)
- New composable: useFormatDate (formatDate, relativeTime, toDateInputValue,
  minutesSince, hoursSince) — extracts duplicated date helpers from
  TaskBoardView and BoardCard
- New composable: useConfirm — reusable confirmation dialog logic
  (open/close, error/success state, Escape binding, body scroll lock)
- New component: StatusPill — unified state pill for backlog/progress/
  review/blocked/done, replacing inline .detail-state-pill classes
- New component: SkeletonLoader — loading placeholder with shimmer
  animation (card/text/circle variants)
- TaskBoardView: imports StatusPill & useFormatDate, removes 35+ lines
  of duplicated helpers
- ui/index.ts: exports new StatusPill & SkeletonLoader
- Build verified: pnpm build green (vue-tsc --noEmit + vite build pass)

frontend/src/composables/useFormatDate.ts

@@ -0,0 +1,60 @@
+/**
+ * useFormatDate — Einheitliche Datums-/Zeitformatierung für Nexus V2
+ *
+ * Kapselt die mehrfach wiederholten formatDate-, relativeTime- und
+ * toDateInputValue-Helper, die bisher in TaskBoardView, BoardCard,
+ * mit anderen Views dupliziert waren.
+ *
+ * Alle Ausgaben orientieren sich an de-DE Locale.
+ */
+
+/**
+ * Datum als lesbaren String (de-DE, kurzes/medium-DateStyle).
+ */
+export function formatDate(date?: string | null, withTime = false): string {
+  if (!date) return '—'
+  return new Date(date).toLocaleString('de-DE', withTime
+    ? { dateStyle: 'medium', timeStyle: 'short' }
+    : { dateStyle: 'medium' })
+}
+
+/**
+ * ISO-Datumsstring in YYYY-MM-DD für <input type="date">.
+ */
+export function toDateInputValue(date?: string | null): string {
+  if (!date) return ''
+  return new Date(date).toISOString().slice(0, 10)
+}
+
+/**
+ * Relative Zeit ("gerade eben", "vor 5 min", "vor 3 h", "vor 2 d").
+ * Optionaler Fallback, wenn date fehlt.
+ */
+export function relativeTime(date?: string | null, fallback = 'keine Aktivität'): string {
+  if (!date) return fallback
+  const diffMs = Date.now() - new Date(date).getTime()
+  const mins = Math.max(0, Math.round(diffMs / 60000))
+  if (mins < 1) return 'gerade eben'
+  if (mins < 60) return `vor ${mins} min`
+  const hours = Math.round(mins / 60)
+  if (hours < 24) return `vor ${hours} h`
+  const days = Math.round(hours / 24)
+  return `vor ${days} d`
+}
+
+/**
+ * Minuten seit einem Datum (oder Infinity falls kein Datum).
+ */
+export function minutesSince(dateStr?: string | null): number {
+  if (!dateStr) return Infinity
+  return (Date.now() - new Date(dateStr).getTime()) / 60000
+}
+
+/**
+ * Stunden seit einem Datum (gerundet).
+ */
+export function hoursSince(dateStr: string): number {
+  const now = Date.now()
+  const then = new Date(dateStr).getTime()
+  return Math.round((now - then) / 3600000)
+}