feat: improve editor performance (#14429)

#### PR Dependency Tree


* **PR #14429** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

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

* **New Features**
* HTML import now splits lines on <br> into separate paragraphs while
preserving inline formatting.

* **Bug Fixes**
* Paste falls back to inserting after the first paragraph when no
explicit target is found.

* **Style**
  * Improved page-mode viewport styling for consistent content layout.

* **Tests**
* Added snapshot tests for <br>-based paragraph splitting; re-enabled an
e2e drag-page test.

* **Chores**
* Deferred/deduplicated font loading, inline text caching,
drag-handle/pointer optimizations, and safer inline render
synchronization.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tests/affine-local/e2e/drag-page.spec.ts

@@ -231,7 +231,7 @@ test('items in favourites can be reordered by dragging', async ({ page }) => {
 });
 
 // some how this test always timeout, so we skip it
-test.skip('drag a page link in editor to favourites', async ({ page }) => {
+test('drag a page link in editor to favourites', async ({ page }) => {
   await clickNewPageButton(page);
   await page.waitForTimeout(500);
   await page.keyboard.press('Enter');