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>

lib/active-sprint.ts

@@ -40,12 +40,20 @@ async function notifyUserSettings(
   `
 }
 
-export async function getActiveSprintIdFromSettings(
-  userId: string,
+type StoredActiveSprintState =
+  | { kind: 'unset' }
+  | { kind: 'cleared' }
+  | { kind: 'set'; sprintId: string }
+
+export function readStoredActiveSprintState(
+  settings: UserSettings,
   productId: string,
-): Promise<string | null> {
-  const settings = await readSettings(userId)
-  return settings.layout?.activeSprints?.[productId] ?? null
+): StoredActiveSprintState {
+  const map = settings.layout?.activeSprints
+  if (!map || !(productId in map)) return { kind: 'unset' }
+  const value = map[productId]
+  if (value === null) return { kind: 'cleared' }
+  return { kind: 'set', sprintId: value }
 }
 
 export async function setActiveSprintInSettings(
@@ -71,10 +79,10 @@ export async function clearActiveSprintInSettings(
   productId: string,
 ): Promise<void> {
   const current = await readSettings(userId)
-  const existing = current.layout?.activeSprints
-  if (!existing || !(productId in existing)) return
-  const nextActiveSprints = { ...existing }
-  delete nextActiveSprints[productId]
+  const nextActiveSprints: Record<string, string | null> = {
+    ...(current.layout?.activeSprints ?? {}),
+    [productId]: null,
+  }
   const next: UserSettings = {
     ...current,
     layout: { ...current.layout, activeSprints: nextActiveSprints },
@@ -89,10 +97,14 @@ export async function resolveActiveSprint(
   productId: string,
   userId: string,
 ): Promise<ActiveSprint | null> {
-  const stored = await getActiveSprintIdFromSettings(userId, productId)
-  if (stored) {
+  const settings = await readSettings(userId)
+  const state = readStoredActiveSprintState(settings, productId)
+
+  if (state.kind === 'cleared') return null
+
+  if (state.kind === 'set') {
     const sprint = await prisma.sprint.findFirst({
-      where: { id: stored, product_id: productId },
+      where: { id: state.sprintId, product_id: productId },
       select: { id: true, code: true, status: true },
     })
     if (sprint) return sprint

lib/user-settings.ts

@@ -45,7 +45,7 @@ const DevToolsPrefs = z.object({
 
 const LayoutPrefs = z.object({
   splitPanePositions: z.record(z.string(), z.array(z.number())).optional(),
-  activeSprints: z.record(z.string(), z.string()).optional(),
+  activeSprints: z.record(z.string(), z.string().nullable()).optional(),
 }).strict()
 
 export const UserSettingsSchema = z.object({