Fix pre-existing react-refresh/only-export-components lint warnings

[gdiab]

Problem

3 ESLint warnings exist in Context files:

• src/context/AuthContext.tsx — exports useAuth alongside AuthProvider
• src/context/ListsContext.tsx — exports useLists alongside ListsProvider
• src/context/ThemeContext.tsx — exports useTheme alongside ThemeProvider

The react-refresh/only-export-components rule flags these because each file exports both a component and a hook.

Proposed Solution

Either:

1. Move each useX hook to a separate file (e.g., src/hooks/useAuth.ts re-exporting from context), or
2. Disable the rule for these specific files with an eslint-disable comment

Option 1 is cleaner but touches more import paths.

Acceptance Criteria

• npm run lint produces 0 warnings
• No regressions in context usage across the app