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/assetfs/layered.go

@@ -9,7 +9,9 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"slices"
 	"sort"
+	"strings"
 	"time"
 
 	"code.gitea.io/gitea/modules/container"
@@ -61,6 +63,8 @@ type LayeredFS struct {
 	layers []*Layer
 }
 
+var _ fs.ReadDirFS = (*LayeredFS)(nil)
+
 // Layered returns a new LayeredFS with the given layers. The first layer is the top layer.
 func Layered(layers ...*Layer) *LayeredFS {
 	return &LayeredFS{layers: layers}
@@ -83,6 +87,27 @@ func (l *LayeredFS) ReadFile(elems ...string) ([]byte, error) {
 	return bs, err
 }
 
+func (l *LayeredFS) ReadDir(name string) (files []fs.DirEntry, _ error) {
+	filesMap := map[string]fs.DirEntry{}
+	for _, layer := range l.layers {
+		entries, err := readDirOptional(layer, name)
+		if err != nil {
+			return nil, err
+		}
+		for _, entry := range entries {
+			entryName := entry.Name()
+			if _, exist := filesMap[entryName]; !exist && shouldInclude(entry) {
+				filesMap[entryName] = entry
+			}
+		}
+	}
+	for _, file := range filesMap {
+		files = append(files, file)
+	}
+	slices.SortFunc(files, func(a, b fs.DirEntry) int { return strings.Compare(a.Name(), b.Name()) })
+	return files, nil
+}
+
 // ReadLayeredFile reads the named file, and returns the layer name.
 func (l *LayeredFS) ReadLayeredFile(elems ...string) ([]byte, string, error) {
 	name := util.PathJoinRel(elems...)