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

Endpoints/SkillEndpoint.cs

@@ -8,6 +8,9 @@ using Gott.Services;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Routing;
+using SixLabors.ImageSharp;
+using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Formats.Webp;
 
 namespace Gott.Endpoints;
 
@@ -31,6 +34,9 @@ public static class SkillEndpoint
         // Skill-Output-Dateien (Bilder etc.)
         app.MapGet("/api/skills/{slug}/output/{filename}", HandleSkillOutput);
 
+        // Thumbnail (gecached)
+        app.MapGet("/api/skills/{slug}/output/{filename}/thumb", HandleSkillOutputThumb);
+
         // Agent ↔ Skill Zuordnung
         app.MapGet("/api/agents/{agentId}/skills", HandleGetAgentSkills);
         app.MapPut("/api/agents/{agentId}/skills", HandleSetAgentSkills);
@@ -135,4 +141,81 @@ public static class SkillEndpoint
         ctx.Response.Headers["Cache-Control"] = "private, max-age=3600";
         await ctx.Response.SendFileAsync(filePath);
     }
+
+    private static readonly HashSet<string> ThumbExtensions = new(StringComparer.OrdinalIgnoreCase)
+    {
+        ".png", ".jpg", ".jpeg", ".gif", ".webp", ".bmp"
+    };
+
+    private static async Task HandleSkillOutputThumb(HttpContext ctx, SkillService skillService)
+    {
+        var slug = ctx.Request.RouteValues["slug"]?.ToString();
+        var filename = ctx.Request.RouteValues["filename"]?.ToString();
+
+        if (string.IsNullOrEmpty(slug) || string.IsNullOrEmpty(filename))
+        {
+            ctx.Response.StatusCode = 400;
+            return;
+        }
+
+        if (filename.Contains("..") || filename.Contains('/') || filename.Contains('\\'))
+        {
+            ctx.Response.StatusCode = 400;
+            return;
+        }
+
+        var ext = Path.GetExtension(filename);
+        if (!ThumbExtensions.Contains(ext))
+        {
+            ctx.Response.StatusCode = 400;
+            await ctx.Response.WriteAsync("Thumbnails nur für Bilder verfügbar");
+            return;
+        }
+
+        var skill = skillService.GetSkill(slug);
+        if (skill == null) { ctx.Response.StatusCode = 404; return; }
+
+        var originalPath = Path.Combine(skill.Path, "output", filename);
+        if (!File.Exists(originalPath)) { ctx.Response.StatusCode = 404; return; }
+
+        // Cache-Verzeichnis: data/cache/skills/{slug}/
+        var dataDir = Path.GetDirectoryName(skillService.SkillsDir);
+        var cacheDir = Path.Combine(dataDir, "cache", "skills", slug);
+        Directory.CreateDirectory(cacheDir);
+
+        var thumbFilename = Path.GetFileNameWithoutExtension(filename) + ".webp";
+        var thumbPath = Path.Combine(cacheDir, thumbFilename);
+
+        // Thumbnail generieren wenn nicht gecacht oder Original neuer
+        if (!File.Exists(thumbPath) || File.GetLastWriteTimeUtc(originalPath) > File.GetLastWriteTimeUtc(thumbPath))
+        {
+            try
+            {
+                using var image = await Image.LoadAsync(originalPath);
+
+                // Max 300px auf der längsten Seite
+                var maxSize = 300;
+                image.Mutate(x => x.Resize(new ResizeOptions
+                {
+                    Size = new Size(maxSize, maxSize),
+                    Mode = ResizeMode.Max
+                }));
+
+                var encoder = new WebpEncoder { Quality = 75 };
+                await image.SaveAsync(thumbPath, encoder);
+            }
+            catch
+            {
+                // Fallback: Original ausliefern wenn Thumbnail-Generierung fehlschlägt
+                ctx.Response.ContentType = MimeTypes.GetValueOrDefault(ext, "application/octet-stream");
+                ctx.Response.Headers["Cache-Control"] = "private, max-age=3600";
+                await ctx.Response.SendFileAsync(originalPath);
+                return;
+            }
+        }
+
+        ctx.Response.ContentType = "image/webp";
+        ctx.Response.Headers["Cache-Control"] = "private, max-age=86400";
+        await ctx.Response.SendFileAsync(thumbPath);
+    }
 }