run_macro: use fetch instead of api.get for /api/file

api.get assumes JSON responses, but /api/file/<name> returns raw
gcode text. The await threw on response.json() and start_pause()
never fired. Use fetch directly and await response.text() to make
sure FileHandler.get's select_file side effect has been processed
before mach.start() runs.

src/js/program-mixin.js

@@ -580,7 +580,18 @@ module.exports = {
         // right file.
         if (file_name != this.state.selected) {
           this.state.selected = file_name;
-          await api.get(`file/${encodeURIComponent(file_name)}`);
+          // GET /api/file/<name> returns gcode text (not JSON), so use
+          // fetch directly. The server's FileHandler.get sets
+          // state.selected as a side effect; we await the response
+          // before starting so mach.start() reads the right file.
+          const resp = await fetch(
+            `/api/file/${encodeURIComponent(file_name)}`,
+            { cache: "no-cache" }
+          );
+          if (!resp.ok) {
+            throw new Error(`file fetch failed: ${resp.status}`);
+          }
+          await resp.text();
         }
         this.load();
         if (this.state.macros[id].alert == true) {