Add static files and HTML templates for web UI

Embedded Tailwind CSS and login/generator templates.
Self-contained with no external dependencies.

internal/templates/templates.go

@@ -0,0 +1,32 @@
+// Package templates provides embedded HTML templates for the web UI.
+package templates
+
+import (
+	"embed"
+	"html/template"
+	"io"
+	"sync"
+)
+
+//go:embed *.html
+var files embed.FS
+
+//nolint:gochecknoglobals // intentional lazy initialization cache
+var (
+	parsed   *template.Template
+	parseOne sync.Once
+)
+
+// Get returns the parsed templates, parsing them on first call.
+func Get() *template.Template {
+	parseOne.Do(func() {
+		parsed = template.Must(template.ParseFS(files, "*.html"))
+	})
+
+	return parsed
+}
+
+// Render renders a template by name to the given writer.
+func Render(w io.Writer, name string, data any) error {
+	return Get().ExecuteTemplate(w, name, data)
+}