refactor: serve site manifest via /assets/site-manifest.json endpoint (#37405)

Slightly reduce the page size for every request, and don't need to use `href="data:`

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <2114189+wxiaoguang@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

services/context/context_template.go

@@ -148,8 +148,7 @@ func (c TemplateContext) HeadMetaContentSecurityPolicy() template.HTML {
 	//    * Maybe this approach should be avoided, don't make the config system too complex, just let users use A
 	return template.HTML(`<meta http-equiv="Content-Security-Policy" content="` +
 		// allow all by default (the same as old releases with no CSP)
-		// "data:" is used to load the manifest in head (maybe also need to be refactored in the future)
-		// maybe some images are also loaded by "data:", need to investigate
+		// maybe some images or markup (external) renders need "data:", need to investigate
 		`default-src * data:;` +
 
 		// enforce nonce for all scripts, disallow inline scripts