Reading Preferences: Fix sizing issues for post detail and web-based comments

[dvdchr]

Part of #22925
Depends on #22938

This PR fixes the sizing issue for the header view, post webview, and comment webview in Reader Detail. This should also resolve sizing issues raised in #22851 (review) and potentially #21797.

• For the header view in Reader Detail, since it's built-in SwiftUI, I've found that there aren't any reliable "hooks" to detect the correct content size. Therefore, I've added a GeometryReader to report for content size changes.
• For the Post and Comment webviews, since ReaderDisplaySetting makes use of the web view's zoom level to increase the content scale, we'd also need to manually calculate the scale after getting the web view's height since it looks like the scrollHeight property does not take zoom level into account.

To test

• Launch the Jetpack app.
• Ensure that the Reader Customization flag is turned on.
• Navigate to the Reader.
• Open any post. Preferably one that has at least 1 comment.
• If you have customized settings, please reset the font and size settings to the default option: Sans for font and Normal for size (right in the middle).
• Open the customization sheet, select 'Mono' font, and move the slider to the rightmost position. Then tap 'Done'.
• 🔎 Verify that:
  • The header view is correctly resized.
  • The Post webview is correctly resized.
  • The Comment webview is correctly resized.

Regression Notes

1. Potential unintended areas of impact
   Should be none. While the web view calculations take ReaderDisplaySetting's scale into account, when the flag is off, the display setting is set to .standard, which has a scale value of 1.0.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manually tested the changes.

3. What automated tests I added (or what prevented me from doing so)
   N/A.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	1 Warning
⚠️	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22939-c559d30
	Version	24.5
	Bundle ID	org.wordpress.alpha
	Commit	c559d30
	App Center Build	WPiOS - One-Offs #9351

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22939-c559d30
	Version	24.5
	Bundle ID	com.jetpack.alpha
	Commit	c559d30
	App Center Build	jetpack-installable-builds #8395

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[wargcm]

LGTM, nice work! 🎉