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/components/tasks/task-detail/task-description.tsx

@@ -180,6 +180,9 @@ export function TaskDescription({ task }: TaskDescriptionProps) {
       </CollapsibleSection>
       {task.constraints ? (
         <CollapsibleSection
+          // Boilerplate identical on every task in the project — always
+          // starts collapsed.
+          defaultOpen={false}
           title={
             <>
               <ShieldAlert className="h-4 w-4 text-amber-600 dark:text-amber-400" />