Test: add unit test suite for frontend components

[Lansa-18]

Closes #19

Feat: Vitest Unit Test Suite for Frontend Workspace (#19)

Description

Adds a full Vitest unit test suite to the Frontend workspace — covering components, pages, and utilities — with coverage enforcement and CI integration.

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Changes

Setup & Configuration

• Installed and configured Vitest as the Frontend test framework (vitest, @testing-library/react, @testing-library/jest-dom, jsdom, @vitejs/plugin-react)
• Created vitest.config.ts with jsdom environment, @/ alias resolution, PostCSS bypass for leaflet's CSS import, and 70% coverage thresholds across all four metrics (lines, statements, functions, branches)
• Added a global test setup file (src/tests/setup.ts) that registers @testing-library/jest-dom matchers
• Added npm run test and npm run test:watch scripts to Frontend/package.json
• Wired Unit tests: npm run test into the frontend CI job between Lint and Build
• Added CLAUDE.md and plan.md to .gitignore to prevent local-only files from being accidentally committed

Tests Written — 44 tests across 8 files

File	Coverage
cn() utility	Core utility function
Badge	All 4 variants
Header	Render and structure
Features	Mobile + desktop viewport branches
AddGistModal	Open/close, submit with fake timers, char limit, Escape key, loading state
Map	Geolocation success + error paths, default markers, modal trigger
Landing page	Smoke test
Map page	Smoke test

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Checklist

• Tests added or updated
• Documentation updated (README, docs, comments)
• Linting passes
• Type checking passes
• Relevant issue linked
• Changelog updated

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Security

No security-sensitive changes. Test coverage improvements reduce the risk of undetected regressions in UI rendering, but no auth, payment, or data-exposure logic was touched.

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Notes for Reviewers

• vitest.config.ts — PostCSS override: css: { postcss: { plugins: [] } } is set to prevent Vite from loading postcss.config.mjs during tests. That file uses @tailwindcss/postcss, which is a Next.js-only plugin and invalid in the Vite/Vitest context. This override only affects the test runner — the Next.js build is unaffected.

• CTA.tsx has no tests: CTA.tsx is not imported by any page or component in the codebase. Testing a dead component would inflate coverage with no behavioral value. Tests should be added when it is wired into a page.

• AddGistModal submit uses fake timers: The submit handler has a hardcoded setTimeout(..., 2000). Tests use vi.useFakeTimers() + vi.advanceTimersByTime(2000) to avoid slow tests.

• To run tests locally:

  cd Frontend
  npm run test          # single run with coverage report
  npm run test:watch    # watch mode

[Lansa-18]

@snowrugar-beep Kindly review the PR.

[snowrugar-beep]

LGTM