2026.9.9-canary.909

fix(editor): preserve expanded headings in readonly mode (#15549)

## Description

Closes #13579.

On readonly shared pages, selecting text under a heading persisted as
collapsed caused reactive selection handling to restore the persisted
collapsed value over the viewer-local expanded state. This closed the
heading again and prevented selecting or copying its revealed content.

This change:
- separates persisted collapsed-state synchronization from selection
cleanup;
- makes the readonly-local collapsed state reactive for both BlockSuite
effects and Lit rendering;
- preserves selection and copying while a readonly heading is locally
expanded;
- still clears hidden text selection when the heading is collapsed
again;
- adds a Playwright regression covering collapse → readonly → expand →
drag-select → copy → re-collapse.

## Checklist

- [x] I have signed the [AFFiNE Contributor License
Agreement](https://cla-assistant.io/toeverything/AFFiNE) — required
before merge; the `license/cla` check must be green ([how it
works](https://github.com/toeverything/AFFiNE/blob/canary/docs/BUILDING.md#sign-the-cla-first))
- [x] The PR targets the `canary` branch and its title follows
[Conventional Commits](https://www.conventionalcommits.org/)
- [x] Tests are added or updated where it makes sense
- [x] `yarn lint` and `yarn typecheck` pass locally

## Testing

- Focused Chromium regression: 1 passed
- Adjacent readonly paragraph scenarios: 3 passed
- Targeted `oxfmt --check`: passed
- Targeted `oxlint --deny-warnings`: passed
- `git diff --check`: passed

`yarn typecheck` could not be confirmed in this checkout because
required generated workspace declaration outputs are missing.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
  * Improved collapsed heading behavior in read-only mode.
* Preserved local heading expansion and text selection when copying
content and collapsing the heading.
* Ensured local expansion state resets correctly when switching out of
and back into read-only mode.

* **Tests**
* Added coverage for read-only heading expansion, keyboard copying,
clipboard access, mode switching, and text selection.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->