add Supabase diagnostics page + automated verification test

[fabcrowd]

• Generates /supabase-test/ at build time from env vars (Netlify or local).
• Adds a Node verification script that checks auth and REST endpoints with required headers.
• Adds a GitHub Action to run the verification using repo secrets.
• You can run locally via npm run verify:supabase. Remove /supabase-test and tools/* when done.

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https://chatgpt.com/codex/tasks/task_e_68e19bc86ef083308e50231545dd883f

[netlify]

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