use replaceQueryParam when changing channels

[devoncarew]

• use GoRouter.replaceQueryParam when changing channels (replaces preserve query params when navigating #2704)
• move the overflow menu to the last item on the toolbar
• show 'external link' icons for the privacy notice and feedback links
• default dartpad to the dark theme (instead of the system one)

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[parlough]

Nice! All these changes work well :)