fix(panel): task-detail tab state in URL, nav placement, kanban overflow, sidebar divider, tooltip sweep

- Task detail: active tab lives in ?tab= (survives reload, back/forward, and
  prev/next task jumps); prev/next arrows move into the header row next to
  Actions instead of their own row above the title
- Constraints section always starts collapsed (project boilerplate)
- Kanban: native overflow scroll replaces Radix ScrollArea (display:table
  viewport let cards grow past the column and clip); columns share width
  (flex-1, 18rem floor, 24rem cap); dark column colors normalized to /40 tints
- Sidebar footer: drop the Separator doubled with the wrapper's border-t
- Tooltips: self-providing Tooltip root (300ms) + hover hints across sidebar,
  header, task detail, kanban, and every icon-only button that had none

panel/src/app/(dashboard)/tasks/[taskId]/page.tsx

@@ -445,14 +445,16 @@ export default function TaskDetailPage({ params }: TaskDetailPageProps) {
 
   return (
     <div className="space-y-6">
-      {/* Breadcrumb (parent task) + prev/next list navigation */}
-      <div className="flex items-center justify-between gap-4">
-        <TaskBreadcrumb task={task} />
-        <TaskListNav task={task} />
-      </div>
+      {/* Breadcrumb (parent task) — renders nothing for a root task */}
+      <TaskBreadcrumb task={task} />
 
-      {/* Header */}
-      <TaskHeader task={task} onAction={handleAction} />
+      {/* Header — prev/next list navigation rides top-right, next to Actions,
+          so it never occupies its own row above the title. */}
+      <TaskHeader
+        task={task}
+        onAction={handleAction}
+        nav={<TaskListNav task={task} />}
+      />
 
       {/* CEO gate #1: Approve & Start a board-reviewed task. After the Board
           finishes, the orchestrator sets board_review_complete and sends the