Improve Supabase auth resilience (offline and failure states)

[YurMil]

Ensure authentication UI (src/components/Auth, auth modal context) degrades gracefully when Supabase is unreachable: clear error messages, no blocked UI, and utilities remain usable anonymously.

Acceptance criteria:

• Network failure during sign-in/sign-up shows a localized, actionable message
• Session refresh failures do not crash pages
• Behavior verified with network throttling/offline emulation

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