Drop files onto a sidebar thread

[sieversjosua]

Problem

When several threads are visible in the sidebar, a file often belongs to a thread other than the one currently open. The workspace drop target added in #6636 only forwards files to the active composer. To attach a screenshot to another thread, the user has to open that thread first and then repeat the drag.

Proposed behavior

While external files are being dragged, each sidebar thread row should become a drop target. Dropping on a row should:

• open that thread;
• pass the files through the existing composer drop path;
• focus the composer; and
• never send anything automatically.

This should not introduce new attachment types or bypass composer validation. Images work today; non-image behavior should remain whatever the composer supports. The interaction must also leave sidebar thread reordering and internal file-tree mention drags unchanged.

Smallest useful scope

Web and desktop sidebar only, for existing threads. No mobile changes, new attachment contracts, or server changes.

Would this be a reasonable small extension of #6636, or do you want file drop targets to remain limited to the active workspace?