Closes #2140
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<!-- CURSOR_SUMMARY -->
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> [!NOTE]
> Implements a chat panel visibility toggle and syncs it with the
resizable layout.
> 
> - Adds `isChatPanelHiddenAtom` to manage chat panel visibility state
> - Adds a header toggle in `PreviewIframe` (with tooltip, icons, and
`data-testid`) to hide/show the chat panel
> - Makes `chat` page's `Panel` for `#chat-panel` collapsible; wires
refs to expand/collapse and syncs with the atom; preserves preview panel
behavior and resize handle
> - Adds Playwright e2e test `e2e-tests/chat_panel_toggle.spec.ts`
verifying collapse/expand behavior
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
d27fd58182f7012ad2b07fb3d1584a07b584bf45. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- This is an auto-generated description by cubic. -->
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## Summary by cubic
Adds a toolbar toggle to hide and show the chat panel, helping users
focus on the preview when needed. The chat panel is now collapsible and
synced with app state and the resize handle, with an e2e test covering
the flow.

- New Features
- Preview toolbar button toggles chat visibility (Maximize2/Minimize2
with tooltip).
- Added isChatPanelHiddenAtom; syncs panel size with hidden state,
restores previous size on re-open, and updates visibility when the
handle is dragged (hidden below ~5% width).
- Playwright test confirms #chat-panel hides/shows via
data-testid="preview-toggle-chat-panel-button".

<sup>Written for commit f41058bfec814dde3d70a53cf20792e9d997f217.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>