Add image lightbox — click gallery image to view full-screen

Clicking any image in a post modal opens it full-screen with a dark
overlay. Close by clicking outside, the ✕ button, or pressing Escape.
Added zoom-in cursor and hover opacity to signal clickability.

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

style.css

@@ -426,7 +426,49 @@ footer {
   object-fit: cover;
   border-radius: 8px;
   flex-shrink: 0;
+  cursor: zoom-in;
+  transition: opacity 0.15s;
 }
+.img-gallery img:hover { opacity: 0.88; }
+
+/* =====================
+   LIGHTBOX
+   ===================== */
+#lightboxOverlay {
+  display: none;
+  position: fixed;
+  inset: 0;
+  background: rgba(0,0,0,0.92);
+  z-index: 9999;
+  align-items: center;
+  justify-content: center;
+  animation: lbFade 0.18s ease;
+}
+#lightboxOverlay.open { display: flex; }
+#lightboxImg {
+  max-width: 92vw;
+  max-height: 90vh;
+  object-fit: contain;
+  border-radius: 6px;
+  box-shadow: 0 8px 40px rgba(0,0,0,0.6);
+}
+.lightbox-close {
+  position: fixed;
+  top: 18px;
+  right: 22px;
+  background: rgba(255,255,255,0.15);
+  border: none;
+  color: #fff;
+  font-size: 1.6rem;
+  width: 42px;
+  height: 42px;
+  border-radius: 50%;
+  cursor: pointer;
+  line-height: 1;
+  transition: background 0.15s;
+}
+.lightbox-close:hover { background: rgba(255,255,255,0.3); }
+@keyframes lbFade { from { opacity:0 } to { opacity:1 } }
 
 /* =====================
    SUBMIT FORM (in modal)