Add unit tests for supabase module

[tomusdrw]

Context

The lib/supabase/ module currently has zero test coverage. All hooks, components, and utilities need tests.

What to test

Hooks (unit tests with mocked context)

• useUser, useSession, useSignOut — throw outside SupabaseProvider, return correct values from context
• useSubscription — returns free/pro status correctly, handles expired subscriptions
• useQuota — returns correct canUse/remaining for free vs pro users, increment updates count
• useUserData — fetch, save, remove operations, app-scoped vs shared data

Components (render tests)

• AuthFlow — renders login/register tabs, switches between them, clears passwords, handles duplicate email detection
• UserMenu — renders login button when logged out, email dropdown when logged in, compact variant
• Settings — renders theme selector
• PricingCard — renders plan info, disables button for active subscribers
• QuotaGate — shows content when under limit, shows pricing card when over, shows progress bar
• SubscriptionStatus — shows free/pro badge, manage/upgrade buttons

Mocking strategy

Use the MockSupabaseProvider from StorybookProvider.tsx (already exists for Storybook) to provide mock context values. For Supabase client calls (.from().select() etc.), mock the client methods.

Acceptance criteria

• All hooks have tests for happy path and error cases
• All components render correctly in both auth states
• Coverage target: 80%+ for lib/supabase/