Add image preview support: Krea-tag parsing with modal, file-explorer preview, skill output endpoint, increase execute timeout to 180s

- SkillEndpoint: GET /api/skills/{slug}/output/{filename} for skill output files
- WorkspaceEndpoint: GET /api/workspace/raw for raw file serving (images)
- File-Explorer: Image preview for png/jpg/gif/webp/svg/bmp/ico
- Chat UI: Parse [Krea: filename] tags into clickable chips with image modal
- ToolService: Increase execute timeout from 30s to 180s for long-running skills

wwwroot/css/file-explorer.css

@@ -105,6 +105,22 @@
   margin: auto;
 }
 
+.fe-image-preview {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  flex: 1;
+  padding: 1em;
+  overflow: auto;
+}
+
+.fe-image-preview img {
+  max-width: 100%;
+  max-height: 100%;
+  object-fit: contain;
+  border-radius: 4px;
+}
+
 
 /* ── CodeMirror / EasyMDE gemeinsame Wrapper-Basis ── */