Portfolio: add filtering, URL sharing, and improved tag input

- Add category filters with dynamic counts that reduce as filters
  applied
- Add ?filter[category]=value URL params for filterable links
- Add ?img= timestamp param that bypasses filters to show specific image
- Update URL when opening/navigating/closing modal for shareable links
- Backend: add /api/portfolio/filters endpoint with filter logic
- Backend: add timestamp lookup fallback for GetImage endpoint
  Frontend:
- Portfolio page: filter dropdowns, keyboard nav, mobile improvements
- ImageUpload: live tag suggestions from API, arrow/Tab navigation,
  confirmation modal before upload, mobile-optimized touch targets
- Add scrollbar-hide utility and fix filter dropdown overflow
- Move Clear all button, add vertical separator on desktop

frontend/src/app.css

@@ -127,4 +127,12 @@
 
 [data-calendar] {
   z-index: 49 !important;
+}
+
+.scrollbar-hide {
+  -ms-overflow-style: none;
+  scrollbar-width: none;
+}
+.scrollbar-hide::-webkit-scrollbar {
+  display: none;
 }