Rework file attachment popover for files, folders, and drag-and-drop

[IgorWarzocha]

Goal

Rework the file attachment popover so attaching files and folders feels reliable, obvious, and native.

Scope

• Improve drag-and-drop behavior in the attachment popover, including clearer drop targets and feedback while dragging.
• Make dropped files and folders attach consistently, instead of depending on fragile current popover behavior.
• Allow clicking a folder action in the popover to attach a folder through the native picker.
• Keep existing file attachment flows working.
• Preserve expected popover behavior: Escape closes it, outside click closes it, and drag/drop interactions should not accidentally close or dismiss it.

Acceptance criteria

• Users can drag files onto the attachment popover and see clear visual feedback before drop.
• Users can drag folders onto the attachment popover and have them attach correctly.
• Users can click a folder option in the popover and select a folder to attach.
• The existing click-to-attach-file flow still works.
• Failed or unsupported drops show a useful error instead of silently doing nothing.
• The implementation has coverage or deterministic validation for the file and folder attachment paths.

Notes

The current file attachment popover is a bit messy and makes folder attachment harder than it should be. This should clean up the interaction model rather than adding another one-off path.