Use heic-convert for HEIC-to-JPEG conversion

Sharp's prebuilt libvips lacks HEIF codec support. Replace with
heic-convert (pure JS decoder) for reliable HEIC conversion on all
platforms. Existing HEIC files on disk will be converted on-the-fly
when served via /api/files.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -12,6 +12,7 @@
   "dependencies": {
     "@types/qrcode": "^1.5.6",
     "framer-motion": "^11.2.0",
+    "heic-convert": "^2.1.0",
     "lucide-react": "^0.400.0",
     "next": "^16.1.6",
     "qrcode": "^1.5.4",