feat(web): accept file drops across the chat workspace

[dbalders]

Problem

Files only attach when they are dropped directly on the composer, so valid drops elsewhere in the active conversation area are easy to miss.

Solution

Make the chat column one continuous file-drop target and show a clear “Drop files to attach” overlay while a file drag is active. Drops are forwarded to the existing composer attachment path, so its supported types, limits, validation errors, and focus behavior remain authoritative.

The target deliberately excludes the window chrome, left sidebar, and separate right panel so their existing drag behavior is unchanged. Text, link, and other non-file drags are ignored. Cancelled drags and thread changes clear the overlay.

This expands the drop target only; it does not add new attachment formats or overlap the separate non-image attachment work in #3927.

Fixes #6474

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[macroscopeapp]

Approvability

Verdict: Approved 79a5f6f

This PR is a self-contained UI enhancement that expands the file drop zone from the composer to the entire chat workspace. It refactors existing drag-drop logic into a reusable module with unit tests, with no backend or behavioral changes beyond the improved UX.

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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[CDVolvik]

workspaceFileDrop.test.ts: 4/4.

Drops were composer-only; whole-workspace drop is what people try first. MERGEABLE. I did not drag a file in the running app.