feat(PBI-79/ST-1333): active-sprint null-contract + clearActiveSprintAction

- lib/user-settings.ts: activeSprints values nullable in Zod-schema.
  Key-aanwezigheid heeft nu betekenis (key+null = bewust geen sprint;
  key ontbreekt = fallback-cascade).
- lib/active-sprint.ts: nieuwe readStoredActiveSprintState helper +
  resolveActiveSprint respecteert expliciet 'cleared' state zonder fallback.
  clearActiveSprintInSettings schrijft null i.p.v. de key te verwijderen.
- actions/active-sprint.ts: nieuwe clearActiveSprintAction met auth +
  membership-check.
- components/shared/sprint-switcher.tsx: '— Geen actieve sprint —'-optie
  in dropdown, disabled wanneer er geen actieve sprint is.
- Tests: nieuwe active-sprint.test.ts (resolver-paden + clear),
  active-sprint-action.test.ts (action-laag), uitbreiding user-settings.test.ts.

Plan: docs/plans/PBI-79-backlog-sprint-workflow.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

components/shared/sprint-switcher.tsx

@@ -13,7 +13,7 @@ import {
   DropdownMenuTrigger,
 } from '@/components/ui/dropdown-menu'
 import { cn } from '@/lib/utils'
-import { setActiveSprintAction } from '@/actions/active-sprint'
+import { clearActiveSprintAction, setActiveSprintAction } from '@/actions/active-sprint'
 import type { SprintStatusApi } from '@/lib/task-status'
 import { debugProps } from '@/lib/debug'
 
@@ -67,6 +67,22 @@ export function SprintSwitcher({
     })
   }
 
+  function handleClearActiveSprint() {
+    if (!activeSprint) return
+    startTransition(async () => {
+      const result = await clearActiveSprintAction(productId)
+      if (result?.error) {
+        toast.error(typeof result.error === 'string' ? result.error : 'Wisselen mislukt')
+        return
+      }
+      if (pathname.includes('/sprint')) {
+        router.push(`/products/${productId}`)
+      } else {
+        router.refresh()
+      }
+    })
+  }
+
   if (sprints.length === 0) {
     return (
       <span {...debugProps('sprint-switcher')}>
@@ -127,6 +143,17 @@ export function SprintSwitcher({
           Toon afgeronde sprints
         </button>
         <DropdownMenuSeparator />
+        <DropdownMenuItem
+          onClick={handleClearActiveSprint}
+          disabled={!activeSprint || isPending}
+          className={cn(
+            'italic text-muted-foreground',
+            !activeSprint && 'opacity-50 cursor-not-allowed',
+          )}
+        >
+          — Geen actieve sprint —
+        </DropdownMenuItem>
+        <DropdownMenuSeparator />
         {visibleSprints.length === 0 ? (
           <div className="px-2 py-2 text-sm text-muted-foreground/70 italic">
             Geen open sprints