feat: Add file upload support in chat

[sre-helmcode]

Summary

Allow users to upload files directly from the chat interface and attach them to messages sent to the agent team.

Current State

• The chat input is a standard HTML <input> element in TeamMonitorPage.tsx (line 622-636)
• The ChatRequest type only accepts { message: string } (types/index.ts, lines 272-274)
• There is no file upload UI component, no <input type="file">, and no FormData handling
• The Settings button (SkillStatusPanel) is the only icon next to the input

Proposed Solution

1. Add a file attachment button (paperclip/attach icon) next to the Settings button in the chat input area
2. Add a hidden <input type="file"> triggered by the attachment button
3. Show file preview (name, size, type) as a chip/pill above the input when a file is attached
4. Allow removing attached files before sending
5. Modify handleSend() to use multipart/form-data when files are attached
6. Supported file types: Common text files (.txt, .md, .json, .yaml, .py, .go, .ts, .js, etc.), images (.png, .jpg), and PDFs
7. File size limit: Define a max file size (e.g., 10MB) with client-side validation

Layout Reference

┌──────────────────────────────────────────────────────┐
│ [📎 Attach] [⚙️ Settings] [textarea...]   [Send]    │
└──────────────────────────────────────────────────────┘

When a file is attached:

┌──────────────────────────────────────────────────────┐
│  📄 config.yaml (2.3 KB)  ✕                         │
├──────────────────────────────────────────────────────┤
│ [📎 Attach] [⚙️ Settings] [textarea...]   [Send]    │
└──────────────────────────────────────────────────────┘

Affected Files

• src/pages/TeamMonitorPage.tsx — Chat input area (lines 613-644)
• src/services/api.ts — chatApi.send() (lines 250-256)
• src/types/index.ts — ChatRequest interface (lines 272-274)

Dependencies

• Requires backend changes: New endpoint or modified POST /api/teams/:id/chat to accept multipart/form-data with file attachments. See companion issue in helmcode/agent_crew_api.

Acceptance Criteria

• Attach button visible next to Settings icon in chat
• Clicking attach opens file picker
• Attached file shown as preview chip with remove option
• File sent with message to backend
• File size validation with user-friendly error
• Supported file types validated
• Works when team is in running state
• Disabled when team is not running (like other chat controls)

[sre-helmcode]

Implemented in 6d96c63. File upload UI in chat: attach button, file preview chips, client-side validation (10MB/5 files/MIME), FormData with backward JSON compat, error recovery.