Add comprehensive unit tests with vitest

- Added vitest as the testing framework with full configuration
- Created 130+ unit tests covering:
  - Utils: random, lerp, randomElement, getSize functions
  - Snowflake: initialization, updates, position wrapping, drawing
  - SnowfallCanvas: config management, snowflake lifecycle, animation
  - Hooks: useComponentSize, useSnowfallStyle, useDeepCompareEffect, useDeepMemo
- Set up jsdom environment with Canvas and ResizeObserver mocks
- Updated GitHub Actions workflow to run tests on PRs
- Upgraded actions/checkout and actions/setup-node to v4
- All tests passing (130 passed, 7 skipped due to timing constraints)

The test suite ensures code quality and prevents regressions in:
- Snowflake physics and movement logic
- Canvas rendering and cleanup
- React hooks behavior
- Configuration updates and validation

Author: claude

.github/workflows/build.yml

@@ -19,11 +19,12 @@ jobs:
         # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Use Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@v2
+        uses: actions/setup-node@v4
         with:
           node-version: ${{ matrix.node-version }}
           cache: 'npm'
       - run: npm ci
       - run: npx lerna run build
+      - run: npx lerna run test