feat(chat): file upload support for web chat

[vybe]

Context

Client feedback (Paradigm Life, April 2026) noted inability to share files for agent analysis via the web interface.

Current State

• Telegram file upload implemented (feat: File upload support for external channels (Telegram, Slack, public links) #354, feat(telegram): file upload support Phase 1 (#354) #355) - extraction, download, validation
• Web chat (AgentDetail, PublicChat) has no file upload capability
• Pattern exists in Telegram adapter that can be reused

Requested

Enable file drag-and-drop and/or file picker in the web chat input.

Acceptance Criteria

• File picker button or drag-drop zone in ChatInput
• Validate file type and size before upload
• Upload to agent workspace (reuse Telegram pattern)
• Pass file reference to agent in chat context
• Visual feedback during upload
• Works in both authenticated and public chat views

Validation Required

• Confirm security model for file uploads (size limits, allowed types)
• Decide: inline in message vs separate upload endpoint
• Consider: how does agent access uploaded files?

Prior Art

• Telegram adapter: _extract_files(), download_file(), magic-byte validation
• Claude.ai: drag-and-drop file attachment

References

• Client feedback: "No file upload or snippet support"
• Effort: Medium (backend pattern exists from Telegram)
• Priority: Medium

/label enhancement,ui,P2

[vybe]

/claim

[github-actions]

Claimed by @vybe. Label status-in-progress added.

Next steps:

1. Create branch: feature/364-<short-slug>
2. Implement the feature
3. Open a PR with Fixes #364