Update Mastra tutorial with working workflow and auth handling (#696)

* Rewrite Mastra tutorial as single-page guide with agent and workflow

- Consolidate multi-page tutorial into single comprehensive page
- Add tool configuration with MCP servers and individual tools
- Include output truncation to prevent token overflow
- Add email digest workflow example
- Update Steps component CSS to not number nested h4-h6 headings
- Add callout linking to multi-user auth guide
- Update redirects to point old pages to new single page

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Update Mastra workflow to use auth handling and Slack DM

- Add auth error handling with arcade.auth.start() in workflow steps
- Pass authRequired/authUrl through workflow steps
- Use Slack_WhoAmI to get user's Slack ID
- Send digest as DM instead of posting to #general channel
- Use defaultUserId from ARCADE_USER_ID env var
- Update documentation to explain workflow auth handling

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Merge origin/main into mastra-tutorial-update

* adding images to the Mastra example

* fix image path

---------

Co-authored-by: Rachel Lee Nabors <nearestnabors@users.noreply.github.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: nearestnabors

app/en/get-started/agent-frameworks/_meta.tsx

@@ -23,7 +23,7 @@ export const meta: MetaRecord = {
     title: "OpenAI Agents",
   },
   vercelai: {
-    title: "Vercel AI",
+    title: "Vercel AI SDK",
   },
 };