refactor: modernize Go idioms across codebase

Replace map[string]interface{} with map[string]any, ioutil.ReadAll with
io.ReadAll, and simplify redundant fmt.Sprintf/w.Write calls.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

reset.go

@@ -16,7 +16,7 @@ func resetPassword(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	base := map[string]interface{}{
+	base := map[string]any{
 		"login": r.URL.Query().Get("l"),
 		"token": r.URL.Query().Get("t"),
 	}