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/conf_server_frontend.go

@@ -7,11 +7,12 @@ import (
 // JSCKServerFrontend is the frontend section's configuration key.
 const JSCKServerFrontend string = "frontend"
 
-// JSCServerFrontend configures how the server serves the frontend. Nothing
-// reads it yet. Dir serves the frontend from disk instead of the embedded
-// build; empty means the embedded build. ImmutablePrefix is where Vite emits
-// content-hashed files, and IndexCacheControl the Cache-Control of HTML and
-// other files without a hash; empty means none.
+// JSCServerFrontend configures how the server serves the frontend. Dir is
+// the directory of the frontend's build output (holding index.html); empty
+// means no frontend is served, as there is no embedded build yet.
+// ImmutablePrefix is where Vite emits content-hashed files, cached for a
+// year; empty means "/assets/". IndexCacheControl is the Cache-Control of
+// HTML and other files without a hash; empty means "no-cache".
 type JSCServerFrontend struct {
 	Dir               string `cfg:"dir"`
 	ImmutablePrefix   string `cfg:"immutable_prefix"`