PROMPT.md-based skill injection, scroll-on-image-load fix, skill availability UI

- SkillService: read PROMPT.md instead of SKILL.md for system prompt injection
- SkillInfo: add HasPromptMd field, skills without PROMPT.md are not injectable
- Agent form: disable checkbox + warning for skills missing PROMPT.md
- Thumbnail size: 320px (was 300px)
- Chat: _scrollOnImages() re-scrolls after lazy-loaded images finish loading
- CSS: skill-unavailable/warning styling, adjusted thumb dimensions

wwwroot/js/ui.js

@@ -79,6 +79,16 @@ function scrollDown() {
   });
 }
 
+function _scrollOnImages(container) {
+  var imgs = container.querySelectorAll('img');
+  for (var i = 0; i < imgs.length; i++) {
+    if (!imgs[i].complete) {
+      imgs[i].addEventListener('load', scrollDown);
+      imgs[i].addEventListener('error', scrollDown);
+    }
+  }
+}
+
 function esc(s) {
   const d = document.createElement('div');
   d.textContent = s;
@@ -234,6 +244,7 @@ function addAssistantBubble(text, timestamp = '', messageId = null, isMarked = f
   if (messageId) div.dataset.messageId = messageId;
   div.innerHTML = renderMarkdown(text);
   wrapKreaGalleries(div);
+  _scrollOnImages(div);
   _addReplyButton(div);
   _addMarkButton(div, messageId, isMarked);
   chatEl().appendChild(div);
@@ -299,6 +310,7 @@ function endStreamBubble() {
     else {
       _streamDiv.innerHTML = renderMarkdown(_streamText);
       wrapKreaGalleries(_streamDiv);
+      _scrollOnImages(_streamDiv);
     }
   }
   _streamDiv = null;