Served the frontend's build output from server.frontend.dir

internal/frontend serves a Vite build from an fs.FS: hashed files under
/assets/ are cached for a year, HTML and other files get no-cache, ETags are
computed (and cached for a build that can't change), precompressed variants
are served when the client accepts them, and a path without an extension
falls back to index.html for client-side routes. Only GET and HEAD are
allowed.

server.WithFrontend serves it for every path outside /api and /ws, behind
compression and the frontend group's hooks. job-search serve passes the
directory in server.frontend.dir, checked for an index.html at start; empty
means no frontend. Its option building moved into serverOptions, with the
access log.

internal/config/README.md

@@ -90,7 +90,7 @@ The `server` section is split into a number of sub-sections:
 | `server.csrf` | CSRF and cross-origin protection settings |
 | `server.tls` | TLS certificate, minimum version and HTTP redirect |
 | `server.headers` | HTTP header settings including policy headers |
-| `server.frontend` | How the frontend is served (not used yet) |
+| `server.frontend` | Where the frontend's build output is and how it is cached |
 | `server.websocket` | Websocket configuration |
 
 #### Server
@@ -177,13 +177,11 @@ The `server` section is split into a number of sub-sections:
 
 #### Frontend
 
-Not used yet.
-
 | Key | Type | Default | Notes |
 |---|---|---|---|
-| `server.frontend.dir` | string | | Serve the frontend from this directory; empty: the embedded build |
-| `server.frontend.immutable_prefix` | string | | Where Vite emits content-hashed files |
-| `server.frontend.index_cache_control` | string | | `Cache-Control` of HTML and other files without a hash; empty: none |
+| `server.frontend.dir` | string | | Directory of the frontend's build output, holding `index.html`; empty: no frontend is served (there is no embedded build yet) |
+| `server.frontend.immutable_prefix` | string | | Where Vite emits content-hashed files, cached for a year; empty: `/assets/` |
+| `server.frontend.index_cache_control` | string | | `Cache-Control` of HTML and other files without a hash; empty: `no-cache` |
 
 #### Websockets