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

Endpoints/SkillEndpoint.cs

@@ -193,8 +193,8 @@ public static class SkillEndpoint
             {
                 using var image = await Image.LoadAsync(originalPath);
 
-                // Max 300px auf der längsten Seite
-                var maxSize = 300;
+                // Max 320px auf der längsten Seite
+                var maxSize = 320;
                 image.Mutate(x => x.Resize(new ResizeOptions
                 {
                     Size = new Size(maxSize, maxSize),