[6.x] Hide default Statamic logo on frontend auth pages

[jasonvarga]

Summary

• The Outside layout is used for auth-style pages rendered both inside the Control Panel (/cp/auth/*) and on the frontend (/!/auth/*, e.g. the default password reset screen).
• On frontend routes we no longer render the default Statamic logo fallback — developers shouldn't see Statamic branding on their own site's auth pages when they haven't customized anything.
• Custom logos (image or text) set via statamic.cp.custom_logo_url / custom_logo_text continue to render on both CP and frontend routes.
• Exposes isCpRoute through the shared Inertia _statamic props so the Vue layout can distinguish the two contexts; hides the entire logo wrapper on frontend routes when nothing would render, to avoid leaving an empty padded block.

Test plan

• Visit /cp/auth/login with no custom logo configured — default Statamic logo still appears.
• Visit /!/auth/password/reset/{token} with no custom logo — no logo is rendered and there's no empty space where it used to be.
• Configure statamic.cp.custom_logo_url — custom logo appears on both /cp/auth/* and /!/auth/*.
• Configure statamic.cp.custom_logo_text — custom text appears on both /cp/auth/* and /!/auth/*.

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