refactor: move tenant templates to internal/tenant

- Move internal/build/ to internal/tenant/
- Rename assets for clarity
- Add tenant-blog.js for shared blog functionality
- Update style.css with improved code block styling

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/build/assets/assets.go

@@ -1,15 +0,0 @@
-package assets
-
-import (
-	"embed"
-	"io/fs"
-	"net/http"
-)
-
-//go:embed css js
-var staticFS embed.FS
-
-func Handler() http.Handler {
-	sub, _ := fs.Sub(staticFS, ".")
-	return http.FileServer(http.FS(sub))
-}