Fix iOS download via Content-Disposition header, fix COI gate null check

- Server: presigned GET URLs now include Content-Disposition: attachment header
  when forDownload=true, triggering native browser downloads on all platforms
- Download button uses window.location.href with attachment URL (works on iOS Safari)
- Bulk download uses hidden iframes instead of fetch+blob
- Fix COI gate: getCOIStatus returns null (not undefined) when undeclared,
  so `!== undefined` was always true — changed to `!= null`

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/(jury)/jury/competitions/[roundId]/projects/[projectId]/evaluate/page.tsx

@@ -398,7 +398,8 @@ export default function JuryEvaluatePage({ params: paramsPromise }: PageProps) {
   const coiRequired = evalConfig?.coiRequired ?? true
 
   // Determine COI state: declared via server or just completed in this session
-  const coiDeclared = coiCompleted || coiStatus !== undefined
+  // coiStatus is null when no COI record exists, truthy when declared
+  const coiDeclared = coiCompleted || (coiStatus != null)
   const coiConflict = coiHasConflict || (coiStatus?.hasConflict ?? false)
 
   // Check if round is active