test(components): cover Footer nav, external rel, and dynamic year

[yunus-dev-codecrafter]

closes #149

## Summary

Adds comprehensive test coverage for the `Footer` component, covering internal nav links, external Discord link safety attributes, and the dynamic copyright year with a mocked `Date`.

## Changes

### `src/components/__tests__/Footer.test.tsx`

- **Footer landmark & tagline** — asserts `<footer role="contentinfo">` renders and the tagline text is present.
- **Internal navigation links** — validates the `<nav aria-label="Footer">` landmark contains four `<Link>` elements (About, Docs, Changelog, Stats) with correct `href` attributes and no `target`/`rel`.
- **Route resolution** — verifies every internal link target resolves to an existing route under `src/app/`.
- **External Discord link safety** — ensures the Discord `<a>` tag carries `target="_blank"` and `rel="noopener noreferrer"`.
- **Dynamic copyright year** — freezes `Date` with `jest.useFakeTimers()` and asserts the copyright line shows the expected year.

### `jest.config.ts`

- Added per-file coverage threshold for `./src/components/Footer.tsx` at 100% across all metrics to prevent regressions.

## Test Results

PASS src/components/tests/Footer.test.tsx
Footer
✓ renders the footer landmark and tagline
✓ renders the navigation landmark with internal links
✓ validates each internal link target resolves to an existing route under src/app
✓ renders the external Discord link safely
✓ renders the dynamic copyright year

File | % Stmts | % Branch | % Funcs | % Lines
Footer.tsx | 100 | 100 | 100 | 100

**Coverage:** 100% on `Footer.tsx` — all statements, branches, functions, and lines covered.

[mikewheeleer]

clean diff — merging now