Add Westgate global footer (#8)

## Summary
- add the global Westgate footer partial under `templates/partials/footer/`
- import scoped footer styles from `theme.scss`
- include the footer design spec and powered-by HTML/CSS references
- add a contract test for the footer integration and required links

## Verification
- `node tests/global-footer-contract.test.js`
- `node tests/global-topbar-contract.test.js`
- `for test in tests/*.test.js; do node "$test"; done`
- `npm test --if-present`
- `git diff --check`

Reviewed-on: #8
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.com>

theme.scss

@@ -2,6 +2,7 @@
 @import "./scss/westgate/tokens";
 @import "./scss/westgate/wiki-prose";
 @import "./scss/westgate/topbar";
+@import "./scss/westgate/footer";
 @import "./scss/westgate/surfaces";
 @import "./scss/westgate/categories";
 @import "./scss/westgate/topics";