Reduce editor keydown plugin work

[ComputelessComputer]

Compute placeholder decorations from the selected block and limit image trailing paragraph repair to changed document ranges so large notes avoid document-wide plugin scans during typing.

---

Note

Medium Risk
Placeholder and image-repair logic changed how empty blocks are targeted; behavior is covered by new tests but still affects all note editing UX.

Overview
Editor typing performance: placeholderPlugin no longer walks the whole document with doc.descendants; it resolves a single empty block from the selection (including nested blocks and gap-cursor neighbors) and applies at most one decoration. imageTrailingParagraphPlugin only inspects images in transaction changed ranges (via getChangedRanges) instead of every top-level child on each edit.

Desktop: ClassicMainBody is wrapped in a memoized ClassicMainBodyHost, and TimelineView is exported as memo(...) so parents can skip redundant subtree work. Timeline tests pass explicit props on rerender where memo would otherwise leave stale UI.

Tests: New Vitest coverage for placeholder selection behavior and image trailing-paragraph insert/repair edge cases.

^{Reviewed by Cursor Bugbot for commit 9cc5fb2. Bugbot is set up for automated code reviews on this repo. Configure here.}

[netlify]

✅ Deploy Preview for old-char ready!

Name	Link
🔨 Latest commit	29a9304
🔍 Latest deploy log	https://app.netlify.com/projects/old-char/deploys/6a488f21fe545500080e6160
😎 Deploy Preview	https://deploy-preview-5906--old-char.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[cursor]

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

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 0238516. Configure here.}