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/css/chat.css

@@ -627,17 +627,13 @@
 
 .krea-thumb {
   display: block;
-  max-width: 300px;
-  max-height: 200px;
+  max-width: 320px !important;
+  max-height: 240px;
   object-fit: contain;
 }
 
 /* Krea Image Modal */
 
-.krea-overlay {
-  backdrop-filter: blur(4px);
-}
-
 .krea-modal {
   max-width: 90vw;
   max-height: 90vh;