feat(portal): shift claiming and my-shifts for volunteer portal

Backend: PortalShiftController with 4 endpoints (available-shifts,
my-shifts, claim, cancel) delegating to ShiftAssignmentService.
24 PHPUnit tests covering happy paths, auth, conflicts, and edge cases.

Frontend: claim-shifts and my-shifts pages with TanStack Query
composable, conflict detection, confirmation dialogs, and cancel flow.
Navigation and dashboard cards wired up for approved volunteers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/portal/src/components/portal/StatusCard.vue

@@ -94,12 +94,12 @@ const registeredLabel = computed(() => {
           sm="4"
         >
           <VCard
-            :to="{ name: 'portal-shifts' }"
+            :to="{ name: 'portal-my-shifts' }"
             variant="outlined"
             class="pa-4 h-100 text-decoration-none"
           >
             <div class="text-subtitle-2 text-medium-emphasis mb-1">
-              Mijn Shifts
+              Mijn Diensten
             </div>
             <div class="text-body-2">
               Rooster bekijken
@@ -111,14 +111,15 @@ const registeredLabel = computed(() => {
           sm="4"
         >
           <VCard
+            :to="{ name: 'portal-claim-shifts' }"
             variant="outlined"
-            class="pa-4 h-100 text-medium-emphasis"
+            class="pa-4 h-100 text-decoration-none"
           >
-            <div class="text-subtitle-2 mb-1">
-              Shifts claimen
+            <div class="text-subtitle-2 text-medium-emphasis mb-1">
+              Diensten claimen
             </div>
             <div class="text-body-2">
-              Binnenkort beschikbaar
+              Schrijf je in voor diensten
             </div>
           </VCard>
         </VCol>