A user-requested drag target should reuse the existing attachment pipeline instead
of creating a second upload path. The composer now handles browser DataTransfer
drops and Tauri native drag-drop events, while keeping desktop attachments
path-only so large files are not serialized into chat payloads. The fallback
/goal metadata is also aligned with the existing desktop command surface because
the desktop gate exercises that menu while validating composer behavior.
Constraint: Desktop attachments must remain path-only to avoid inflated websocket payloads
Rejected: Read dropped desktop files with FileReader | would reintroduce large inline data payloads
Confidence: high
Scope-risk: moderate
Directive: Do not replace the Tauri drag-drop path conversion with data URLs without rerunning payload-size regression coverage
Tested: cd desktop && bun run test -- --run src/lib/composerAttachments.test.ts src/components/chat/ChatInput.test.tsx src/pages/EmptySession.test.tsx
Tested: cd desktop && bun run lint
Tested: cd desktop && bun run build
Tested: bun run check:desktop
Tested: agent-browser overlay and dropped-chip smoke at http://127.0.0.1:1421/?serverUrl=http%3A%2F%2F127.0.0.1%3A3456
Tested: bun run verify
Not-tested: Finder-to-packaged-.app manual drag smoke
