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/components/dashboard/v2/AgentNode.vue

@@ -157,7 +157,7 @@ defineEmits<{
 
 .nc-av.iris-av {
   background: var(--grad);
-  color: var(--tx);
+  color: #fff;
   box-shadow: var(--glow-purple);
 }
 
@@ -255,7 +255,7 @@ defineEmits<{
 }
 
 .node.is-work .nc-bar i {
-  background: linear-gradient(90deg, var(--st-work), var(--st-work));
+  background: linear-gradient(90deg, #2bb87f, #3ddc97);
 }
 
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