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.

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This PR was written with the help of Claude Opus 4.6

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Signed-off-by: silverwind <[email protected]>
Co-authored-by: Claude (Opus 4.6) <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>