Reader: Fix navigation button animation issues

[dvdchr]

Refs #22770

This addresses two issues:

• When transitioning to "Subscriptions", the navigation button starts from a shifted position.
• The filter chip buttons should animate from below the navigation button.

The first one is particularly tricky. It turned out that animating the label component of SwiftUI.Menu makes it twitch. The workaround is to separate the container capsule view and its contents with a ZStack container.

•	Before	After
Filter chip animates from below the navigation button
Navigation button shouldn't twitch during transition	before-480.mov	after-480.mov

To test

• Launch the Jetpack app and navigate to the Reader tab.
• Change streams and observe the transition between chips, especially when transitioning to/from "Subscriptions".
• 🔎 Verify that the transitions work as described.

Regression Notes

1. Potential unintended areas of impact
   Should be none.

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)

[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	pr22808-98fb22f
	Version	24.4
	Bundle ID	com.jetpack.alpha
	Commit	98fb22f
	App Center Build	jetpack-installable-builds #8159

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[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	pr22808-98fb22f
	Version	24.4
	Bundle ID	org.wordpress.alpha
	Commit	98fb22f
	App Center Build	WPiOS - One-Offs #9116

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wargcm]

LGTM! 🚀