Fix theme discovery and Vite dev server in dev mode (#37033)

1. In dev mode, discover themes from source files in
`web_src/css/themes/` instead of AssetFS. In prod, use AssetFS only.
Extract shared `collectThemeFiles` helper to deduplicate theme file
handling.
2. Implement `fs.ReadDirFS` on `LayeredFS` to support theme file
discovery.
3. `IsViteDevMode` now performs an HTTP health check against the vite
dev server instead of only checking the port file exists. Result is
cached with a 1-second TTL.
4. Refactor theme caching from mutex to atomic pointer with time-based
invalidation, allowing themes to refresh when vite dev mode state
changes.
5. Move `ViteDevMiddleware` into `ProtocolMiddlewares` so it applies to
both install and web routes.
6. Show a `ViteDevMode` label in the page footer when vite dev server is
active.
7. Add `/__vite_dev_server_check` endpoint to vite dev server for the
health check.
8. Ensure `.vite` directory exists before writing the dev-port file.
9. Minor CSS fixes: footer gap, navbar mobile alignment.

---
This PR was written with the help of Claude Opus 4.6

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/base/footer_content.tmpl

@@ -4,17 +4,22 @@
 			<a target="_blank" href="https://about.gitea.com">{{ctx.Locale.Tr "powered_by" "Gitea"}}</a>
 		{{end}}
 		{{if (or .ShowFooterVersion .PageIsAdmin)}}
+			<span>
 			{{ctx.Locale.Tr "version"}}:
 			{{if .IsAdmin}}
 				<a href="{{AppSubUrl}}/-/admin/config">{{AppVer}}</a>
 			{{else}}
 				{{AppVer}}
 			{{end}}
+			</span>
 		{{end}}
 		{{if and .TemplateLoadTimes ShowFooterTemplateLoadTime}}
-			{{ctx.Locale.Tr "page"}}: <strong>{{LoadTimes .PageStartTime}}</strong>
-			{{ctx.Locale.Tr "template"}}{{if .TemplateName}} {{.TemplateName}}{{end}}: <strong>{{call .TemplateLoadTimes}}</strong>
+			<span>
+				{{ctx.Locale.Tr "page"}}: <strong>{{LoadTimes .PageStartTime}}</strong>
+				{{ctx.Locale.Tr "template"}}{{if .TemplateName}} {{.TemplateName}}{{end}}: <strong>{{call .TemplateLoadTimes}}</strong>
+			</span>
 		{{end}}
+		{{if $.ViteModeIsDev}}<span class="ui basic label primary">ViteDevMode</span>{{end}}
 	</div>
 	<div class="right-links" role="group" aria-label="{{ctx.Locale.Tr "aria.footer.links"}}">
 		<div class="ui dropdown custom" id="footer-theme-selector">