fix(desktop): treat dropped folders as context refs

[DaveCharland]

Summary

• Detect File-bearing OS drops that resolve to directories before treating them as file uploads.
• Attach local dropped folders as folder context refs instead of routing them through file.attach.
• Show an actionable warning for local folder drops while connected to a remote gateway, where the remote host cannot read the local folder path.

Why

Dragging a folder from the OS can arrive in the renderer with a native File handle. The previous split treated every File-bearing drop as a file/image attachment candidate. For folders, that eventually called file.attach with a directory path and no uploaded bytes, producing the gateway error:

file not found on gateway and no data_url provided

The fix probes the resolved path with the Desktop filesystem bridge before the file/image branch. If the path is a directory, the composer handles it as folder context.

Test plan

npm run test:ui -- src/app/chat/hooks/use-composer-actions.test.ts
# 8 tests passed

npm run typecheck
# tsc -p . --noEmit

[teknium1]

Thanks for the focused folder-drop investigation.

Automated hermes-sweeper review found this behavior is already implemented on current main:

• a488fcf107d1c61400db4274c0baf357b908c772 (merged as fix(desktop): detect dropped folders so they attach as @folder refs #56029) detects OS-dropped directories synchronously through DataTransferItem.webkitGetAsEntry().
• apps/desktop/src/app/chat/hooks/use-composer-actions.ts:165-205 emits detected directories as path-only entries, and :527-539 routes those entries to folder context attachments rather than file.attach.
• apps/desktop/src/app/chat/hooks/use-composer-actions.test.ts:124-187 covers directory classification, exclusion from the upload path, and duplicate items/files handling.

That commit is contained in release v2026.7.1. Closing as implemented on main.

[DaveCharland]

Thanks Tec! You the man. Sorry again to hear about your bestie Link. I'm sure he and my precious angel Minnie and waiting for us on the other side of the rainbow bridge.