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/cli/serve/command.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"log"
 	"log/slog"
 	"os"
 	"os/signal"
@@ -16,19 +15,23 @@ import (
 	"github.com/spf13/cobra"
 
 	"git.avsdev.uk/Go/job-search/internal/cli"
+	"git.avsdev.uk/Go/job-search/internal/config"
 	"git.avsdev.uk/Go/job-search/internal/events"
 	"git.avsdev.uk/Go/job-search/internal/fetcher"
+	"git.avsdev.uk/Go/job-search/internal/frontend"
 	"git.avsdev.uk/Go/job-search/internal/logging"
 	"git.avsdev.uk/Go/job-search/internal/server"
 )
 
-// NewCommand returns the serve command, which serves the REST API and the
-// WebSocket endpoint, and runs a worker for every enabled source.
+// NewCommand returns the serve command, which serves the REST API, the
+// WebSocket endpoint and the frontend, and runs a worker for every enabled
+// source.
 func NewCommand(opts *cli.Options) *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "serve",
-		Short: "Serve the REST API (/api) and WebSocket (/ws), and fetch from sources",
-		Long: `Serve the REST API under /api and the WebSocket endpoint under /ws.
+		Short: "Serve the REST API, WebSocket and frontend, and fetch from sources",
+		Long: `Serve the REST API under /api, the WebSocket endpoint under /ws, and the
+frontend's files from server.frontend.dir under / if it is set.
 
 It also runs a worker for every enabled source, as fetcher service does,
 saving what they fetch to database.file if it is set, and pushing an
@@ -82,23 +85,13 @@ set, the API serves the stored opportunities under /api/opportunities.`,
 				conf.Server.AccessLog = accessLogOverride
 			}
 
-			var options []server.Option
-			if conf.Server.AccessLog != "" {
-				var (
-					accessLog      *log.Logger
-					closeAccessLog func() error
-				)
-				accessLog, closeAccessLog, err = logging.OpenAccessLog(
-					conf.Server.AccessLog,
-				)
-				if err != nil {
-					return err
-				}
-				defer func() {
-					err = errors.Join(err, closeAccessLog())
-				}()
-				options = append(options, server.WithAccessLog(accessLog))
+			options, closeAccessLog, err := serverOptions(&conf.Server)
+			if err != nil {
+				return err
 			}
+			defer func() {
+				err = errors.Join(err, closeAccessLog())
+			}()
 
 			// ctx is cancelled on the first SIGINT (Ctrl+C) or SIGTERM.
 			ctx, stop := signal.NotifyContext(
@@ -201,3 +194,38 @@ set, the API serves the stored opportunities under /api/opportunities.`,
 
 	return cmd
 }
+
+// serverOptions returns the server's options for conf: the access log, if
+// server.access_log is set, and the frontend's files, if server.frontend.dir
+// is (checked first). closeAccessLog closes the access log; call it once the
+// server has stopped.
+func serverOptions(
+	conf *config.JSCServer,
+) ([]server.Option, func() error, error) {
+	var options []server.Option
+	closeAccessLog := func() error {
+		return nil
+	}
+	if conf.AccessLog != "" {
+		accessLog, closeFile, err := logging.OpenAccessLog(conf.AccessLog)
+		if err != nil {
+			return nil, nil, err
+		}
+		closeAccessLog = closeFile
+		options = append(options, server.WithAccessLog(accessLog))
+	}
+
+	if dir := conf.Frontend.Dir; dir != "" {
+		files := os.DirFS(dir)
+		err := frontend.CheckFS(files)
+		if err != nil {
+			return nil, nil, errors.Join(
+				fmt.Errorf("server.frontend.dir %s: %w", dir, err),
+				closeAccessLog(),
+			)
+		}
+		options = append(options, server.WithFrontend(files))
+	}
+
+	return options, closeAccessLog, nil
+}