fix(web): a column drag no longer re-renders the app or re-lays out an embedded document per pixel

[agentforce314]

What

Follow-up to #936, for the resize cursor that lingered after a release. Two things made a right-column drag heavy on a full page:

• The shell and the conversation root subscribed to the column's width itself, so every pixel of a drag re-rendered the whole app. They now subscribe to whether the column is open, which changes only when it opens or closes.
• An HTML preview's frame (or a PDF) followed the column pixel by pixel, laying its document out again each time, and a page of wide tables can take longer per pixel than a frame lasts. While a handle is held the frame keeps the width it had; the release lays it out once, at the final width.
• The width preference is persisted once the writes settle rather than on every frame.

Verification

• ui-web: npm run typecheck clean; vitest run 50 files, 631 tests passing, with tests for the published drag state, the open/closed store changing only on open and close, and the frame's width freezing and thawing.
• In the browser, a synthetic twelve-step drag over the HTML preview handled each move in under a millisecond, produced no long frames, and cleared the drag state three milliseconds after the release, with the frame frozen for the gesture.

🤖 Generated with Claude Code

https://claude.ai/code/session_018nGcaynrdG2uR3d2r1EczY