Krea inline thumbnails with cached WebP generation and gallery layout

- Add /api/skills/{slug}/output/{filename}/thumb endpoint with ImageSharp
- Generate 300px WebP thumbnails, cached in data/cache/skills/{slug}/
- Replace text badge with inline thumbnail preview in chat
- Click opens full-size image modal with backdrop blur
- wrapKreaGalleries() groups consecutive krea-refs into flex container
- Thumbnails lazy-loaded, cache invalidated when original is newer

wwwroot/css/chat.css

@@ -605,32 +605,39 @@
 
 /* Krea Image Refs in Assistant Bubbles */
 
-.krea-ref-chip {
-  display: inline-flex;
-  align-items: center;
-  gap: .3em;
-  padding: .15em .5em;
-  background: var(--f2-bg);
+.krea-gallery {
+  display: flex;
+  flex-wrap: wrap;
+  gap: var(--badge-spacing);
+  margin: var(--box-spacing) 0;
+}
+
+.krea-ref {
+  display: inline-block;
+  cursor: pointer;
   border: var(--border-width) solid var(--f2-border);
   border-radius: var(--border-radius);
-  font-size: var(--font-size);
-  white-space: nowrap;
-  vertical-align: baseline;
-  cursor: pointer;
-  transition: background var(--transition-duration), border-color var(--transition-duration);
+  overflow: hidden;
+  vertical-align: middle;
 }
 
-.krea-ref-chip:hover {
-  border-color: var(--f3-border);
-  background: var(--f3-bg);
+.krea-ref:hover {
+  border-color: var(--primary);
 }
 
-.krea-ref-chip .fa {
-  color: var(--primary);
+.krea-thumb {
+  display: block;
+  max-width: 300px;
+  max-height: 200px;
+  object-fit: contain;
 }
 
 /* Krea Image Modal */
 
+.krea-overlay {
+  backdrop-filter: blur(4px);
+}
+
 .krea-modal {
   max-width: 90vw;
   max-height: 90vh;