feat: add attachment / image / video upload button beside chat input field

[Al629176]

Summary

Add a file attachment button beside the chat input field so users can upload images, videos, or files directly into the conversation without needing to paste URLs or use workarounds.

Current State

The chat input field currently supports text input only. There is no affordance for attaching files, images, or videos alongside a message.

Expected Behaviour

• An attachment/paperclip icon (or image icon) is placed beside the chat input field
• Clicking it opens a file picker allowing the user to select:
  • Images (PNG, JPG, GIF, WebP, etc.)
  • Videos (MP4, MOV, etc.)
  • Files/documents (PDF, text, code files, etc.)
• Selected file is shown as a preview thumbnail or filename chip in the input area before sending
• File is sent with the message and visible in the conversation thread
• Agent can read/analyse the attached content (image vision, document parsing) as part of its response

Scope

• Attachment button UI beside the input field (consistent with existing input styling)
• File picker — images, videos, documents
• Preview chip / thumbnail shown in input area before sending
• File included in the message payload sent to the agent
• Agent can reference and process the attached content (vision for images, text extraction for PDFs)
• Drag-and-drop onto the chat input area as an alternative to clicking the button
• Paste from clipboard (Ctrl/Cmd+V) to attach an image directly

Acceptance Criteria

• Attachment icon is visible and accessible beside the chat input on the Chat page
• User can select an image/video/file and see a preview before sending
• Attached file is sent with the message
• Agent can see and respond to the attachment content
• Works on macOS desktop (Tauri)