Rewrite infinite scroll to use card-based virtual scrolling

- Measure page positions via first card's getBoundingClientRect
  (page wrappers use display:contents so have no box)
- Convert individual cards to placeholders when scrolling away
  (store HTML in jQuery data, set fixed dimensions, empty content)
- Restore cards from placeholders when scrolling back
- Calculate current page based on which page's top is closest
  to viewport bottom without being below it
- DLP (Desired Loaded Pages) = [CP-2, CP-1, CP, CP+1, CP+2]
- Load one page at a time via AJAX

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

wp-content/themes/homeproz/template-parts/property/property-card.scss

@@ -8,6 +8,7 @@
     display: flex;
     flex-direction: column;
     height: 100%;
+    max-height: 525px;
     background-color: var(--color-bg-dark);
     border: 1px solid var(--color-accent);
     border-radius: 0.5rem;

wp-content/themes/homeproz/template-parts/property/property-filters.scss

@@ -659,6 +659,13 @@
     display: contents;
 }
 
+// Placeholder cards maintain grid position when content is unloaded
+.property-card.is-placeholder {
+    background: var(--color-surface, #1a1a1a);
+    border: 1px solid var(--color-border, #333);
+    border-radius: 0.5rem;
+}
+
 // Placeholders maintain scroll height when pages unloaded
 .infinite-scroll-placeholder {
     grid-column: 1 / -1;