Make connection test URL editable

[TurboTheTurtle]

Summary:

• Add a public setting for the connection test URL.
• Preserve https://cp.cloudflare.com as the initialized default.
• Use the configured URL when running the HTTPS connectivity test.
• Add the setting to both mobile and TV settings.

Why:
Some networks cannot reach the default connectivity endpoint. Letting users choose the URL makes the connection test useful in those environments without changing proxy behavior.

Closes #3225.

Validation:

• git diff --check
• JAVA_HOME=/opt/homebrew/opt/openjdk@21 ANDROID_HOME=/Users/andy/Library/Android/sdk ./gradlew :mobile:compileDebugKotlin 📺compileGoogleDebugKotlin
• JAVA_HOME=/opt/homebrew/opt/openjdk@21 ANDROID_HOME=/Users/andy/Library/Android/sdk ./gradlew :mobile:installDebug
• Emulator smoke: launched the debug app, opened Settings > Advanced, and verified Connection test URL shows https://cp.cloudflare.com.

Please preserve author attribution if this PR is squashed or reworked:
Co-authored-by: Andy Ye 35905412+TurboTheTurtle@users.noreply.github.com