Add file attachments to chat

[yikkuro]

Summary

• add a native multi-file picker with main-process MIME detection and base64 encoding
• thread optional attachments through preload, chat IPC, the renderer store, and OpenClaw chat.send
• add removable composer previews, image thumbnails, attachment-only sends, and transcript attachment rendering
• localize per-file, aggregate, picker, and read errors in English and Chinese

Limits

• 20 MiB per file
• 50 MiB aggregate per message, including files already pending
• invalid or oversized files are rejected individually where possible

Validation

• desktop: npx vitest run src/chat-attachments.test.ts src/gateway-client.test.ts (25 tests)
• renderer: npx vitest run src/stores/chat.test.ts (67 tests)
• desktop: npx tsc --noEmit
• renderer: npx vue-tsc --noEmit
• renderer: npm run build

Closes #57