refactor: use combined shareId (locationId + checksum) in URL

Changes:
- Add generateShareId() and extractShareId() helpers
- Share URLs now use single parameter: /share/location/{shareId}
- shareId = locationId (24 chars) + checksum (16 chars) = 40 chars total
- Update validateShareAccess() to extract locationId from shareId
- Update uploadProofOfPayment() to accept combined shareId
- Update LocationViewPage to validate and extract locationId from shareId

Benefits:
- Simpler URL structure (one parameter instead of two)
- Checksum extraction by length (deterministic, no parsing needed)
- Same security properties (HMAC-SHA256 validation)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/[locale]/share/location/[id]/LocationViewPage.tsx

@@ -1,14 +1,28 @@
 import { ViewLocationCard } from '@/app/ui/ViewLocationCard';
-import { fetchLocationById, setSeenByTenantAt } from '@/app/lib/actions/locationActions';
+import { fetchLocationById, setSeenByTenantAt, validateShareAccess } from '@/app/lib/actions/locationActions';
 import { getUserSettingsByUserId } from '@/app/lib/actions/userSettingsActions';
 import { notFound } from 'next/navigation';
 import { myAuth } from '@/app/lib/auth';
 
-export default async function LocationViewPage({ locationId }: { locationId:string }) {
+export default async function LocationViewPage({ shareId }: { shareId: string }) {
+    // Validate share access (checks checksum + TTL, extracts locationId)
+    const accessValidation = await validateShareAccess(shareId);
+
+    if (!accessValidation.valid || !accessValidation.locationId) {
+        return (
+            <div className="alert alert-error">
+                <p>{accessValidation.error || 'This content is no longer shared'}</p>
+            </div>
+        );
+    }
+
+    const locationId = accessValidation.locationId;
+
+    // Fetch location
     const location = await fetchLocationById(locationId);
 
     if (!location) {
-        return(notFound());
+        return notFound();
     }
 
     // Fetch user settings for the location owner
@@ -23,5 +37,11 @@ export default async function LocationViewPage({ locationId }: { locationId:stri
          await setSeenByTenantAt(locationId);
     }
 
-    return (<ViewLocationCard location={location} userSettings={userSettings} />);
+    return (
+        <ViewLocationCard
+            location={location}
+            userSettings={userSettings}
+            shareId={shareId}
+        />
+    );
 }

app/[locale]/share/location/[id]/page.tsx

@@ -3,12 +3,11 @@ import LocationViewPage from './LocationViewPage';
 import { Main } from '@/app/ui/Main';
 import { LocationEditFormSkeleton } from '@/app/ui/LocationEditForm';
 
-export default async function Page({ params:{ id } }: { params: { id:string } }) {
-
+export default async function Page({ params: { id } }: { params: { id: string } }) {
     return (
         <Main>
             <Suspense fallback={<LocationEditFormSkeleton />}>
-                <LocationViewPage  locationId={id} />
+                <LocationViewPage shareId={id} />
             </Suspense>
         </Main>
     );