Sanitize artifact ID input to hex-only characters

- Convert input to lowercase
- Strip non-hex characters (a-f, 0-9 only)
- Limit to 64 characters (SHA256 length)
- Show character count in validation hint

frontend/src/pages/PackagePage.tsx

@@ -365,7 +365,7 @@ function PackagePage() {
           <input
             type="text"
             value={artifactIdInput}
-            onChange={(e) => setArtifactIdInput(e.target.value)}
+            onChange={(e) => setArtifactIdInput(e.target.value.toLowerCase().replace(/[^a-f0-9]/g, '').slice(0, 64))}
             placeholder="Enter SHA256 artifact ID (64 hex characters)"
             className="artifact-id-input"
           />
@@ -383,7 +383,7 @@ function PackagePage() {
           </a>
         </div>
         {artifactIdInput.length > 0 && artifactIdInput.length !== 64 && (
-          <p className="validation-hint">Artifact ID must be exactly 64 hex characters</p>
+          <p className="validation-hint">Artifact ID must be exactly 64 hex characters ({artifactIdInput.length}/64)</p>
         )}
       </div>