feat(board): rework quick-peek modal (Linear-style) + board toolbar

Modal redesign — compact summary instead of two long cards:
- Topbar: breadcrumb + state pill + copyable task-id/link chip; review
  actions (approve / request changes) live in the modal header too
- Main column: title, meta chips (ball, created, last update), compact
  description, then square summary tiles (done/total with progress,
  active, open, blocked) + per-agent pills — details stay in Vollansicht
- Right sidebar: properties as slim Linear-style rows (status, priority,
  assignee, due, source) with hover-reveal controls; activity moved next
  to properties as a compact timeline with a new comment composer
  (POST tasks/{id}/activity via new store action postTaskActivity)
- Unified button system (32px: primary/ghost/approve/changes/icon)
- flattenBoard includes nested children again so quick-peek works for
  child rows inside master cards

Board (issue-tracker basics):
- Toolbar with search (title/detail/id/child titles) and ball filters:
  Alle / Du bist dran / Bei Iris / Haengt

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/stores/tasks.ts

@@ -321,6 +321,18 @@ export const useTaskStore = defineStore('tasks', {
       }
     },
 
+    /* ── API: Kommentar/Aktivität an Task posten ──── */
+    async postTaskActivity(id: string, message: string, type = 'comment') {
+      const res = await apiFetch(`/api/dashboard/tasks/${id}/activity`, {
+        method: 'POST',
+        body: JSON.stringify({ message, type }),
+      })
+      if (!res.ok) {
+        const body = await res.json().catch(() => ({}))
+        throw new Error(body.error || `HTTP ${res.status}`)
+      }
+    },
+
     /* ── API: Fetch agent workflow overview ──────── */
     async fetchAgentOverview(staleHours = 2) {
       this.agentOverviewLoading = true