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]/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>
     );