Fixes anthropics#19

Add file upload functionality to the chat window and display uploaded files.

* **File Upload Handling:**
  - Add file upload functionality to `customer-support-agent/app/page.tsx`.
  - Implement file handling logic for text, image, and PDF files.
  - Display uploaded files and their contents in the chat window.
  - Add progress indicators for large file uploads.

* **Chat Area Updates:**
  - Update `customer-support-agent/components/ChatArea.tsx` to handle file uploads and display file previews inline.
  - Integrate file preview components from `financial-data-analyst/components/FilePreview.tsx`.
  - Add expandable file preview functionality.

* **Right Sidebar Updates:**
  - Add a separate file section for uploaded files in `customer-support-agent/components/RightSidebar.tsx`.
  - Display thumbnails or icons for each uploaded file.
  - Open a modal with a preview when clicking on a file.

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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/anthropics/anthropic-quickstarts/issues/19?shareId=XXXX-XXXX-XXXX-XXXX).