Embedded the frontend in the binary, and built it in the build script

`npm run sync` in web/ copies the build output (web/dist, from npm run build)
to internal/frontend/dist, which the binary embeds. Go can't embed from
outside the package's directory (web/ is also outside the Go module), hence
the copy. Only dist/.gitkeep is committed, so a fresh clone still compiles,
and a binary built without syncing first serves no frontend.

job-search serve serves server.frontend.dir if it is set, or else the
embedded build, or warns that there is no frontend. scripts/build.sh builds
and syncs the frontend before building, running npm ci first for a release
or when web/node_modules is missing.

internal/cli/serve/command.go

@@ -31,7 +31,9 @@ func NewCommand(opts *cli.Options) *cobra.Command {
 		Use:   "serve",
 		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.
+frontend under /: the files in server.frontend.dir if it is set, or else the
+build embedded in the binary (copied there by npm run sync in web/ before
+building), if there is one.
 
 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
@@ -196,9 +198,10 @@ set, the API serves the stored opportunities under /api/opportunities.`,
 }
 
 // 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.
+// server.access_log is set, and the frontend's files: those in
+// server.frontend.dir if it is set (checked first), or else the build
+// embedded in the binary, if there is one. closeAccessLog closes the access
+// log; call it once the server has stopped.
 func serverOptions(
 	conf *config.JSCServer,
 ) ([]server.Option, func() error, error) {
@@ -225,7 +228,15 @@ func serverOptions(
 			)
 		}
 		options = append(options, server.WithFrontend(files))
+		return options, closeAccessLog, nil
 	}
 
+	files := frontend.Embedded()
+	if files == nil {
+		slog.Warn("no frontend embedded and server.frontend.dir not set")
+		return options, closeAccessLog, nil
+	}
+	options = append(options, server.WithFrontend(files))
+
 	return options, closeAccessLog, nil
 }