fix: use mime.TypeByExtension for static asset Content-Type

http.DetectContentType() cannot detect JS, CSS, SVG, etc. from magic
bytes — it falls back to text/plain, causing browsers to reject module
scripts with 'Expected a JavaScript module' error.

Use mime.TypeByExtension(ext) which correctly maps .js ->
application/javascript, .css -> text/css, etc.

internal/api/router.go

@@ -1,7 +1,9 @@
 package api
 
 import (
+	"mime"
 	"net/http"
+	"path"
 
 	"github.com/gin-gonic/gin"
 	"github.com/llamalink/llamalink/internal/api/handlers"
@@ -84,7 +86,12 @@ func New(cfg *config.Config, db *gorm.DB, llamaManager *llama.Manager) *gin.Engi
 			c.String(http.StatusNotFound, "asset not found")
 			return
 		}
-		c.Data(http.StatusOK, http.DetectContentType(data), data)
+		ext := path.Ext(filepath)
+		contentType := mime.TypeByExtension(ext)
+		if contentType == "" {
+			contentType = "application/octet-stream"
+		}
+		c.Data(http.StatusOK, contentType, data)
 	})
 	r.GET("/admin", func(c *gin.Context) {
 		index, err := web.Index()