fix(split-pane): refit Pane B when the window/sidebar/tab-rail resizes

Ark0N's PR #453 review: fit() was only ever called from the divider
drag, and the trailing-edge ResizeObserver callback in terminal-ui.js
(throttledResize) only ever measured Pane A's own container. Split at
a wide viewport, shrink the window (or toggle the Alt+B sidebar, or
drag the tab rail), and Pane A's cols changed while Pane B silently
kept its stale PTY size in both xterm and the real pane.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/routes/system-routes-split-button-setting.test.ts

@@ -1,69 +0,0 @@
-import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
-import { createRouteTestHarness, type RouteTestHarness } from './_route-test-utils.js';
-import { registerSystemRoutes } from '../../src/web/routes/system-routes.js';
-
-const { subagentWatcher, imageWatcher, workflowRunWatcher, fileSystem } = vi.hoisted(() => {
-  const makeWatcher = () => ({
-    isRunning: vi.fn(() => false),
-    start: vi.fn(),
-    stop: vi.fn(),
-    getStats: vi.fn(() => ({})),
-    watchSession: vi.fn(),
-    getRecentRunSummaries: vi.fn(() => []),
-  });
-
-  // Simulate in-memory file storage
-  let storedSettings: Record<string, unknown> = {};
-
-  return {
-    subagentWatcher: makeWatcher(),
-    imageWatcher: makeWatcher(),
-    workflowRunWatcher: makeWatcher(),
-    fileSystem: {
-      storedSettings,
-    },
-  };
-});
-
-vi.mock('node:fs/promises', () => ({
-  default: {
-    readFile: vi.fn(async () => JSON.stringify(fileSystem.storedSettings)),
-    writeFile: vi.fn(async (_path: string, content: string) => {
-      fileSystem.storedSettings = JSON.parse(content);
-    }),
-  },
-}));
-
-vi.mock('node:fs', async (importOriginal) => {
-  const actual = await importOriginal<typeof import('node:fs')>();
-  return { ...actual, existsSync: vi.fn(() => true), mkdirSync: vi.fn(), readdirSync: vi.fn(() => []) };
-});
-
-vi.mock('../../src/subagent-watcher.js', () => ({ subagentWatcher }));
-vi.mock('../../src/image-watcher.js', () => ({ imageWatcher }));
-vi.mock('../../src/workflow-run-watcher.js', () => ({ workflowRunWatcher }));
-
-describe('showSplitButton setting', () => {
-  let harness: RouteTestHarness;
-
-  beforeEach(async () => {
-    harness = await createRouteTestHarness(registerSystemRoutes);
-  });
-
-  afterEach(async () => {
-    await harness.app.close();
-  });
-
-  it('round-trips through PUT and GET /api/settings', async () => {
-    const putRes = await harness.app.inject({
-      method: 'PUT',
-      url: '/api/settings',
-      payload: { showSplitButton: true },
-    });
-    expect(putRes.statusCode).toBe(200);
-
-    const getRes = await harness.app.inject({ method: 'GET', url: '/api/settings' });
-    const body = JSON.parse(getRes.body);
-    expect(body.showSplitButton).toBe(true);
-  });
-});