feat(ri): hide the navigation until the pages it links to exist

[ashleythedeveloper]

This PR hides the reference implementation's navigation behind a SHOW_NAVIGATION flag, so users no longer see a sidebar and header whose destination pages are stubs.
The public header, the protected sidebar (desktop and mobile), and all their wiring remain in the codebase; reinstating the navigation later is a one-flag flip per layout.

Closes #715

Test plan

• Public pages render children and footer without the header, with the AppBar top offset removed
• Protected pages render without the sidebars, keeping the auth redirect, loading state, and DID context
• The absence tests fail if the navigation is rendered again (verified by flipping the flag)
• The verify page error-state helper no longer requires an unrelated button to exist (it previously matched the header menu button)

[gemini-code-assist]

Code Review

This pull request introduces a SHOW_NAVIGATION flag set to false in both the protected and public layouts to temporarily hide navigation components (sidebars and header) until their target pages exist. The layout test files have been updated or added to verify this behavior and ensure children render correctly without the navigation elements. There are no review comments, so I have no feedback to provide.

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