refactor: use go:embed for templates

Templates are now embedded using //go:embed and parsed once at startup
with template.Must(template.ParseFS(...)). This avoids re-parsing
template files from disk on every request and removes the dependency
on template files being present at runtime.

closes #7

templates/templates.go

@@ -0,0 +1,8 @@
+package templates
+
+import (
+	"embed"
+)
+
+//go:embed *.html
+var Templates embed.FS