Serve the frontend's config.js with the API password

The bundled frontend gets a token with API_PASSWORD from config.js. The server now
generates config.js from its own config, so setting API_PASSWORD on the server
container is enough.

Dockerfile

@@ -18,8 +18,6 @@ RUN --mount=type=cache,target=/go/pkg/mod --mount=type=cache,target=/root/.cache
 
 # Stage 2: Frontend static files (the same for every platform)
 FROM --platform=$BUILDPLATFORM ${FRONTEND_IMAGE} AS frontend
-# Empty API_HOSTNAME makes the frontend use relative paths
-RUN echo "window.__RUNTIME_CONFIG__ = { API_HOSTNAME: '' };" > /usr/share/nginx/html/config.js
 
 # Stage 3: Final image. No RUN steps here, so no emulation is needed for other architectures.
 FROM alpine:3.22

internal/server/routes.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"encoding/json"
 	"net/http"
 
 	"music-server/cmd/web"
@@ -66,6 +67,7 @@ func (s *Server) routes() (*echo.Echo, apis) {
 	e.GET("/assets/*", echo.WrapHandler(http.FileServer(http.FS(web.Assets))))
 	e.GET("/search", echo.WrapHandler(templ.Handler(web.SearchForm())))
 	e.POST("/find", echo.WrapHandler(web.NewSearchHandler(s.search)))
+	e.GET("/config.js", s.frontendConfig)
 	e.Static("/", s.cfg.FrontendPath)
 
 	for _, r := range e.Router().Routes() {
@@ -74,6 +76,21 @@ func (s *Server) routes() (*echo.Echo, apis) {
 	return e, a
 }
 
+// frontendConfig serves the runtime config of the bundled frontend. The frontend uses
+// the API password to get a token, so anyone who can open the frontend can read it.
+func (s *Server) frontendConfig(c *echo.Context) error {
+	cfg, err := json.Marshal(map[string]string{
+		// Empty: the frontend calls this server with relative paths.
+		"API_HOSTNAME": "",
+		"API_PASSWORD": s.cfg.APIPassword,
+	})
+	if err != nil {
+		return err
+	}
+	c.Response().Header().Set("Cache-Control", "no-store")
+	return c.Blob(http.StatusOK, "text/javascript; charset=utf-8", []byte("window.__RUNTIME_CONFIG__ = "+string(cfg)+";\n"))
+}
+
 func redirect(to string) echo.HandlerFunc {
 	return func(c *echo.Context) error {
 		return c.Redirect(http.StatusMovedPermanently, to)

internal/server/routes_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"net"
 	"net/http"
+	"path/filepath"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -142,6 +143,17 @@ func TestSearchPageAndFrontend(t *testing.T) {
 	assert.Contains(t, rec.Body.String(), "frontend")
 }
 
+func TestFrontendConfigHasAPIPassword(t *testing.T) {
+	e := newTestEnv(t, func(c *config.Config) { c.APIPassword = `pa'ss"word` })
+	writeFile(t, filepath.Join(e.cfg.FrontendPath, "config.js"), "stale")
+
+	rec := e.do(http.MethodGet, "/config.js")
+	assert.Equal(t, http.StatusOK, rec.Code)
+	assert.Equal(t, "text/javascript; charset=utf-8", rec.Header().Get("Content-Type"))
+	assert.Equal(t, "no-store", rec.Header().Get("Cache-Control"))
+	assert.Equal(t, `window.__RUNTIME_CONFIG__ = {"API_HOSTNAME":"","API_PASSWORD":"pa'ss\"word"};`+"\n", rec.Body.String())
+}
+
 func TestShutdownWaitsForRequestsBeforeClosingDatabase(t *testing.T) {
 	e := newTestEnv(t)
 	started := make(chan struct{})