feat: run self-hosted without Supabase credentials (no-account previe…

[yakew7]

feat: run self-hosted without Supabase credentials (no-account preview mode)

Closes #72. Previously the app crashed on every route when Supabase env
vars were unset, because proxy.ts (and the client/server factories) called
createServerClient/createBrowserClient unconditionally, which throws
"Your project's URL and Key are required to create a Supabase client!".

Add an isSupabaseConfigured() gate and degrade cleanly when it's false:

• supabase/client.ts + server.ts return null instead of throwing
• proxy.ts skips session refresh; auth/callback redirects home
• navbar hides sign in/out; /login shows a preview-mode notice
• map saved-places layer and calendar personal events stay hidden
• user-places/user-events CRUD guarded behind requireClient()

The public map, filters, search, and calendar work with zero Supabase
setup, so contributors can preview their map entries locally before
opening a PR. With credentials present, auth and private features are
unchanged.

[vercel]

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