Embed the PWA build and serve it with SPA fallback

Add internal/frontend, which embeds internal/frontend/dist and serves it
with Cache-Control "no-cache" for index.html, sw.js and
manifest.webmanifest and "public, max-age=31536000, immutable" for every
other asset. Any other GET or HEAD falls back to index.html, except paths
under the reserved /api, /auth, /media and /health prefixes, which get
the JSON not_found error instead.

NewRouter now takes the asset fs.FS so tests use a fstest.MapFS fixture,
mounts an /api subrouter with JSON 404/405 handlers, and fails fast if
the assets lack index.html. A placeholder dist/index.html is tracked so a
fresh clone compiles; built assets remain git-ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/server/main.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/config"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/frontend"
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/server"
 )
 
@@ -37,9 +38,13 @@ func run(cfg config.Config, logger *slog.Logger) error {
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer stop()
 
+	router, err := server.NewRouter(logger, version, frontend.Assets())
+	if err != nil {
+		return err
+	}
 	srv := &http.Server{
 		Addr:              cfg.HTTPAddr,
-		Handler:           server.NewRouter(logger, version),
+		Handler:           router,
 		ReadHeaderTimeout: 10 * time.Second,
 		ErrorLog:          slog.NewLogLogger(logger.Handler(), slog.LevelError),
 	}