feat(FN-4077): fix mobile switcher trigger sizing in ChatView

Fixes the mobile switcher trigger sizing in ChatView by updating CSS dimensions and adjusting the component's conditional rendering logic, with corresponding test coverage added.

Fusion-Task-Id: FN-4077

Fusion-Task-Lineage: 966eafb9-35fd-4ac6-ab4a-42c0e96a2584

packages/core/src/__tests__/store-test-helpers.ts

@@ -2,6 +2,7 @@ import { mkdtempSync } from "node:fs";
 import { rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
+import { setTimeout as delay } from "node:timers/promises";
 import { vi } from "vitest";
 
 vi.mock("node:child_process", async (importOriginal) => {
@@ -52,9 +53,10 @@ export function createTaskStoreTestHarness() {
     afterEach: async () => {
       vi.useRealTimers();
       store.stopWatching();
-      // Yield one microtask tick without relying on process.nextTick,
-      // which can be faked in timer-heavy suites and hang teardown.
-      await Promise.resolve();
+      // Yield one real event-loop turn so fs.watch cleanup settles before
+      // close()/rm() run. Promise.resolve() is only a microtask and has proven
+      // too weak for some full-suite watcher teardowns.
+      await delay(0);
       store.close();
       await rm(rootDir, { recursive: true, force: true, maxRetries: 5, retryDelay: 50 });
       await rm(globalDir, { recursive: true, force: true, maxRetries: 5, retryDelay: 50 });