feat(legal): add legal pages and global footer

Introduce Privacy Policy, Terms of Service, and Disclaimers views
Add site footer with copyright, legal links, and attribution notices
Update system wordings with comprehensive legal content in EN/ZH

frontend/src/components/AppShell.vue

@@ -30,6 +30,7 @@ const emit = defineEmits<{
 
 const { t } = useI18n();
 const route = useRoute();
+const copyrightYear = new Date().getFullYear();
 const languageMenu = ref<HTMLElement | null>(null);
 const languageMenuButton = ref<HTMLButtonElement | null>(null);
 const languageMenuOpen = ref(false);
@@ -210,5 +211,19 @@ onBeforeUnmount(() => {
     <main class="page">
       <slot></slot>
     </main>
+
+    <footer class="site-footer">
+      <div class="site-footer__inner">
+        <p class="site-footer__copyright">
+          {{ t('legal.footer.copyright', { year: copyrightYear }) }}
+        </p>
+        <nav class="site-footer__links" :aria-label="t('legal.footer.linksLabel')">
+          <RouterLink to="/privacy-policy" @click="closeSidebar">{{ t('legal.footer.privacy') }}</RouterLink>
+          <RouterLink to="/terms-of-service" @click="closeSidebar">{{ t('legal.footer.terms') }}</RouterLink>
+          <RouterLink to="/disclaimers" @click="closeSidebar">{{ t('legal.footer.disclaimers') }}</RouterLink>
+        </nav>
+        <p class="site-footer__notice">{{ t('legal.footer.notice') }}</p>
+      </div>
+    </footer>
   </div>
 </template>