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/server/routes.go

@@ -7,6 +7,7 @@ import (
 	"github.com/go-chi/chi/v5"
 
 	"git.avsdev.uk/Go/job-search/internal/config"
+	"git.avsdev.uk/Go/job-search/internal/frontend"
 	"git.avsdev.uk/Go/job-search/internal/httpx"
 	"git.avsdev.uk/Go/job-search/internal/middleware"
 )
@@ -18,6 +19,7 @@ import (
 //	  /api : NoStore -> Compress -> [api rate limit + hooks] -> CSRF
 //	         -> RequireJSON
 //	  /ws  : [ws rate limit + hooks] -> Origin check (hub)
+//	  /*   : Compress -> [frontend hooks] -> static files (if WithFrontend)
 func (s *Server) routes() (http.Handler, error) {
 	conf := s.conf
 	router := chi.NewRouter()
@@ -67,6 +69,20 @@ func (s *Server) routes() (http.Handler, error) {
 	webSocketStack := s.groupStack(GroupWebSocket, conf.WebSocket.ConnectionLimit)
 	router.With(webSocketStack...).Get("/ws", s.hub.ServeHTTP)
 
+	// Frontend
+	if s.frontend != nil {
+		frontendHandler := frontend.New(s.frontend, frontend.Options{
+			ImmutablePrefix:   conf.Frontend.ImmutablePrefix,
+			IndexCacheControl: conf.Frontend.IndexCacheControl,
+			// An embedded build never changes; a directory on disk may.
+			CacheETags: conf.Frontend.Dir == "",
+		})
+		frontendStack := append(
+			[]Middleware{compress}, s.hooks[GroupFrontend]...,
+		)
+		router.With(frontendStack...).Handle("/*", frontendHandler)
+	}
+
 	return router, nil
 }