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>

modules/web/middleware/data.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"time"
 
+	"code.gitea.io/gitea/modules/public"
 	"code.gitea.io/gitea/modules/reqctx"
 	"code.gitea.io/gitea/modules/setting"
 )
@@ -36,5 +37,6 @@ func CommonTemplateContextData() reqctx.ContextData {
 		"PageStartTime":      time.Now(),
 
 		"RunModeIsProd": setting.IsProd,
+		"ViteModeIsDev": public.IsViteDevMode(),
 	}
 }