test(playwright): add agent onboarding and inbox creation UI tests (#14707)

Adds E2E UI tests for two core Phase 2 flows, building on the Playwright
setup from #13578.

**Agent onboarding** — validates the Agents settings page, Add Agent
modal elements, form validation (name + email required, submit disabled
until valid), and cancel behaviour.

**Inbox creation** — walks through the full API channel inbox creation
journey: channel selection → form fill → agent assignment → finish
screen.

## What changed

New UI component objects (`tests/playwright/components/ui/`):
- `agent-page.component.ts`
- `add-agent-modal.component.ts`
- `add-agents-form.component.ts`
- `settings-inbox-page.component.ts`
- `channel-selector.component.ts`
- `api-channel-form.component.ts`
- `finish-setup.component.ts`

New test specs (`tests/playwright/tests/e2e/ui/`):
- `agent-onboarding-flow-ui-validation.spec.ts`
- `inbox-creation-flow.spec.ts`

Updated `components/ui/index.ts` barrel export to include all new
components.

## How to test

```bash
cd tests/playwright
npx playwright test tests/e2e/ui/
```

All 5 tests pass locally (3 login + 2 new flows).

Closes part of the Phase 2 scope from the [Playwright E2E discussion
#13500](https://github.com/orgs/chatwoot/discussions/13500).

---------

Co-authored-by: Sony Mathew <sony@chatwoot.com>
Co-authored-by: Sony Mathew <2040199+sony-mathew@users.noreply.github.com>

tests/playwright/components/ui/channel-selector.component.ts

@@ -0,0 +1,25 @@
+import { Page } from '@playwright/test';
+
+export class ChannelSelector {
+  constructor(private page: Page) {}
+
+  getPageHeading() {
+    return this.page.getByRole('heading', { name: /choose channel/i });
+  }
+
+  getApiChannelCard() {
+    return this.page.getByRole('button', { name: /API.*Make a custom channel/i });
+  }
+
+  getWebsiteChannelCard() {
+    return this.page.getByRole('button', { name: /Website.*Create a live-chat widget/i });
+  }
+
+  async selectApiChannel() {
+    await this.getApiChannelCard().click();
+  }
+
+  async selectWebsiteChannel() {
+    await this.getWebsiteChannelCard().click();
+  }
+}