feat: support file upload

[sharanyavinod]

• Introduces support for adding files via file picker/dnd
• Supports images, pdfs and markdown
• Currently only supports upload and using ref in the page. For reading intent from attachment i.e. reading a brief/plan from a markdown and continuing, would need changes on the agent side -> todo
• Files are sent as base64 on first upload; on approval continuations, contentUrl from the content_upload tool result is used instead to avoid retransmitting bytes - takes effect once the companion da-agent PR is deployed

Preview: https://da.live/canvas?nx=ew-upload&nxver=2#/exp-workspace/frescopa/index

Note: adobe-rnd/da-agent#27 needs to be reviewed/merged first

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