Upgrade remaining dependencies and set up standalone ESLint

[devin-ai-integration]

Summary

Session D of the 4-session parallel migration. Upgrades remaining dependencies and sets up standalone ESLint configuration (flat config format) to replace CRA's eslint-config-react-app.

Dependency Upgrades

• react-icons: ^4.3.1 → ^5.6.0
• react-countdown: ^2.3.2 → ^2.3.6 (supports React >= 15, compatible with React 19)
• sass: ^1.49.9 → ^1.99.0
• classnames: ^2.3.1 → ^2.5.1

ESLint Setup

• Installed eslint@^9, @eslint/js@^9, eslint-plugin-react@^7, eslint-plugin-react-hooks@^5 as dev dependencies
• Created eslint.config.js using ESLint flat config format with:
  • @eslint/js recommended rules
  • eslint-plugin-react recommended rules (with react-in-jsx-scope and prop-types disabled)
  • eslint-plugin-react-hooks recommended rules
  • Browser globals configured
  • Test files excluded from linting
• Removed eslintConfig field from package.json
• Added "lint": "eslint src/" script to package.json

Lint Fixes

• Fixed unused variable _ → () in src/lib/words.js (line 17)

Review & Testing Checklist for Human

• Verify npx eslint src/ passes with no errors
• Verify npm run build still works with upgraded deps
• Spot-check that react-icons v5 icons render correctly (imports unchanged, API compatible)

Notes

• react-countdown latest (2.3.6) has peerDependencies: { react: '>= 15' } so it's fully compatible with React 19
• eslint-plugin-react-hooks@^5.2.0 was chosen over v7 to avoid React Compiler rules that aren't applicable yet
• The eslint.config.js uses ESM import syntax; Node emits a warning about missing "type": "module" in package.json but this is harmless and adding it would break CRA's CommonJS expectations

Link to Devin session: https://app.devin.ai/sessions/b0c8fd91fe6847539ab3b779b5a0b23e
Requested by: @shahmir-masood

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