feat: add past-due payment banner to dashboard

Show an amber banner to logged-in users whose grace_period_until is set,
linking to the Stripe Customer Portal to update their card.

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

resources/views/partials/past-due-banner.blade.php

@@ -0,0 +1,18 @@
+@auth
+    @if (auth()->user()->grace_period_until !== null)
+        <div class="mb-4 flex items-center justify-between gap-4 rounded-xl border border-amber-500/40 bg-amber-500/10 p-4 text-amber-900 dark:text-amber-100">
+            <div class="flex-1 text-sm">
+                <strong class="font-semibold">We couldn't charge your card.</strong>
+                Update your payment method by
+                {{ auth()->user()->grace_period_until->format('l, j M') }}
+                or your paid features will end.
+            </div>
+            <a
+                href="{{ route('billing.portal') }}"
+                class="shrink-0 rounded-lg bg-amber-600 px-3 py-1.5 text-sm font-medium text-white hover:bg-amber-700"
+            >
+                Update card
+            </a>
+        </div>
+    @endif
+@endauth