fix(email-renderer): stack AMP columns at mobile widths

packages/email-renderer/test/email.test.ts

@@ -62,13 +62,18 @@ describe("email renderer", () => {
       const draft=structuredClone(document);
       const content=draft.root.slots.content[0]!.slots.content;
       draft.root.slots.content[0]!.slots.content=[{id:"columns",type:"layout.columns",version:1,props:{},styles:{},slots:{columns:Array.from({length:count},(_,index)=>({id:`column-${index}`,type:"layout.column",version:1,props:{width:`${100/count}%`},styles:{},slots:{content:index===0?content:[]}}))}}];
-      const result=await renderEmail(draft,{mjml:true});
+      const result=await renderEmail(draft,{mjml:true,amp:true});
       expect(result.ok).toBe(true);
       if(!result.ok)continue;
       const mjml=result.value.exports!.mjml!;
       expect(mjml.match(/<mj-column\b/g)).toHaveLength(count);
       expect(await validateMjml(mjml)).toEqual([]);
       expect(mjml).toContain("Hello &lt;world&gt;");
+      const amp=result.value.parts.amp;
+      expect(amp?.validation.valid).toBe(true);
+      expect(amp?.content.match(/class="wrn-column"/g)).toHaveLength(count);
+      expect(amp?.content).toContain('class="wrn-columns"');
+      expect(amp?.content).toContain('@media only screen and (max-width:480px){.email-body .wrn-columns{display:block}.email-body .wrn-column{display:block;width:100%}}');
     }
   });
   test("global brand logo survives HTML, AMP, and strict MJML export", async () => {