Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/components/shared/Footer.tsx

@@ -0,0 +1,88 @@
+import { Flex } from '@mantine/core';
+import { useTranslation } from 'react-i18next';
+import { useCookieConsent } from '@app/hooks/useCookieConsent';
+
+interface FooterProps {
+  privacyPolicy?: string;
+  termsAndConditions?: string;
+  accessibilityStatement?: string;
+  cookiePolicy?: string;
+  impressum?: string;
+  analyticsEnabled?: boolean;
+}
+
+export default function Footer({
+  privacyPolicy = 'https://www.stirling.com/legal/privacy-policy',
+  termsAndConditions = 'https://www.stirling.com/legal/terms-of-service',
+  accessibilityStatement = 'accessibility',
+  analyticsEnabled = false
+}: FooterProps) {
+  const { t } = useTranslation();
+  const { showCookiePreferences } = useCookieConsent({ analyticsEnabled });
+
+  return (
+    <div style={{
+      height: 'var(--footer-height)',
+      backgroundColor: 'var(--mantine-color-gray-1)',
+      borderTop: '1px solid var(--mantine-color-gray-2)',
+      display: 'flex',
+      flexDirection: 'column',
+      justifyContent: 'center',
+    }}>
+        <Flex gap="md"
+          justify="center"
+          align="center"
+          direction="row"
+          style={{ fontSize: '0.75rem' }}>
+          <a
+            className="footer-link px-3"
+            id="survey"
+            target="_blank"
+            rel="noopener noreferrer"
+            href="https://stirlingpdf.info/s/cm28y3niq000o56dv7liv8wsu"
+          >
+            {t('survey.nav', 'Survey')}
+          </a>
+          {privacyPolicy && (
+            <a
+              className="footer-link px-3"
+              target="_blank"
+              rel="noopener noreferrer"
+              href={privacyPolicy}
+            >
+              {t('legal.privacy', 'Privacy Policy')}
+            </a>
+          )}
+          {termsAndConditions && (
+            <a
+              className="footer-link px-3"
+              target="_blank"
+              rel="noopener noreferrer"
+              href={termsAndConditions}
+            >
+              {t('legal.terms', 'Terms and Conditions')}
+            </a>
+          )}
+          {accessibilityStatement && (
+            <a
+              className="footer-link px-3"
+              target="_blank"
+              rel="noopener noreferrer"
+              href={accessibilityStatement}
+            >
+              {t('legal.accessibility', 'Accessibility')}
+            </a>
+          )}
+          {analyticsEnabled && (
+            <button
+              className="footer-link px-3"
+              id="cookieBanner"
+              onClick={showCookiePreferences}
+            >
+              {t('legal.showCookieBanner', 'Cookie Preferences')}
+            </button>
+          )}
+        </Flex>
+    </div>
+  );
+}