fix(FN-1082): harden mailbox modal mobile layout

- Add flex-column base layout for .mailbox-modal and make .mailbox-content grow/scroll correctly
- Introduce a dedicated <=768px mailbox mobile CSS section for header, tabs, detail view, and FAB spacing
- Add MailboxModal regression tests that assert required CSS rules and detail-view structural hooks
- Increase batch GitHub issue import route test timeout to 30s for slower retry paths in CI

packages/dashboard/src/routes.test.ts

@@ -3903,7 +3903,7 @@ describe("POST /github/issues/batch-import", () => {
     expect(res.body.results[0].error).toContain("rate limit");
     expect(res.body.results[0].retryAfter).toBe(1);
     expect(fetchSpy.mock.calls.length).toBeGreaterThanOrEqual(4);
-  }, 15000); // Increase timeout for multiple retries
+  }, 30000); // Retry path can exceed 15s in CI/load-constrained environments
 
   it("processes issues sequentially (not parallel)", async () => {
     vi.useFakeTimers();