fix: serve dashboard preview images via /dashboard/{id}/preview.{ext}

app.py

@@ -86,6 +86,16 @@ async def dashboard_render(request: Request, theme_id: str):
     """)
 
 
+@app.get("/dashboard/{theme_id}/preview.{ext}")
+async def dashboard_preview(theme_id: str, ext: str):
+    from models.dashboard import BUILTIN_DASHBOARDS_DIR, USER_DASHBOARDS_DIR
+    for base in [BUILTIN_DASHBOARDS_DIR, USER_DASHBOARDS_DIR]:
+        path = base / theme_id / f"preview.{ext}"
+        if path.exists():
+            return FileResponse(path)
+    raise HTTPException(404)
+
+
 @app.get("/scene/{scene_id}", response_class=HTMLResponse)
 async def scene_render(request: Request, scene_id: str):
     from models.scene import scene_manager