feat(frontend): add /unit-test command for generating tests

- Add Claude command to analyse files and generate unit tests
- Include example test for isFreeEmailDomain utility
- Follow existing test patterns (Jest, it.each, edge cases)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: talissoncosta

frontend/.claude/commands/unit-test.md

@@ -0,0 +1,61 @@
+---
+description: Analyse a frontend file and generate unit tests (Jest)
+---
+
+Generate unit tests for the frontend file at `$ARGUMENTS`.
+
+**Note:** This command is for frontend code only. For backend (Python) tests, see `../api/README.md`.
+
+## Steps
+
+1. **Check for existing tests**
+   - Look for `__tests__/{filename}.test.ts` in the same directory
+   - If tests exist, analyse them for coverage gaps
+
+2. **Read and analyse the source file**
+   - Identify all exported functions, classes, and constants
+   - Note dependencies and imports
+   - Determine testability:
+     - Pure functions (no side effects) → highly testable
+     - React components → may need mocking
+     - Functions with external dependencies → note what needs mocking
+
+3. **Generate test file**
+   - Follow the pattern in `common/utils/__tests__/format.test.ts`
+   - Location: `{sourceDir}/__tests__/{filename}.test.ts`
+   - Use path aliases for imports (`common/...`, `components/...`)
+
+4. **Test structure requirements**
+   - Use `describe` block for each exported function
+   - Use `it.each` for table-driven tests when function has multiple input/output cases
+   - Include edge cases: `null`, `undefined`, empty strings, empty arrays
+   - Include boundary cases where applicable
+
+5. **After generating, run the tests**
+   ```bash
+   npm run test:unit -- --testPathPatterns={filename}
+   ```
+
+## Test file pattern
+
+```typescript
+import { functionName } from 'common/path/to/file'
+
+describe('functionName', () => {
+  it.each`
+    input        | expected
+    ${value1}    | ${result1}
+    ${value2}    | ${result2}
+    ${null}      | ${expectedForNull}
+    ${undefined} | ${expectedForUndefined}
+  `('functionName($input) returns $expected', ({ input, expected }) => {
+    expect(functionName(input)).toBe(expected)
+  })
+})
+```
+
+## Reference
+
+See existing tests at:
+- `common/utils/__tests__/format.test.ts`
+- `common/utils/__tests__/featureFilterParams.test.ts`

frontend/common/utils/__tests__/isFreeEmailDomain.test.ts

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+import isFreeEmailDomain from 'common/utils/isFreeEmailDomain'
+
+describe('isFreeEmailDomain', () => {
+  it.each`
+    input                      | expected
+    ${'user@gmail.com'}        | ${true}
+    ${'user@yahoo.com'}        | ${true}
+    ${'user@hotmail.com'}      | ${true}
+    ${'user@outlook.com'}      | ${true}
+    ${'user@company.com'}      | ${false}
+    ${'user@flagsmith.com'}    | ${false}
+    ${'user@enterprise.co.uk'} | ${false}
+    ${null}                    | ${false}
+    ${undefined}               | ${false}
+    ${''}                      | ${false}
+    ${'invalid-email'}         | ${false}
+    ${'@gmail.com'}            | ${true}
+  `('isFreeEmailDomain($input) returns $expected', ({ expected, input }) => {
+    expect(isFreeEmailDomain(input)).toBe(expected)
+  })
+})