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/styles/main.css

@@ -413,6 +413,47 @@ svg {
   padding: 30px var(--page-padding-x) 58px;
 }
 
+.site-footer {
+  grid-column: 2;
+  width: min(100%, var(--container));
+  margin: 0 auto;
+  padding: 0 24px 34px;
+}
+
+.site-footer__inner {
+  display: grid;
+  gap: 10px;
+  padding-top: 18px;
+  border-top: 1px solid var(--line);
+  color: var(--muted);
+  font-size: 14px;
+}
+
+.site-footer__copyright,
+.site-footer__notice {
+  margin: 0;
+}
+
+.site-footer__copyright {
+  color: var(--ink-soft);
+  font-weight: 850;
+}
+
+.site-footer__links {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 8px 14px;
+}
+
+.site-footer__links a {
+  color: var(--pokemon-blue-deep);
+  font-weight: 850;
+}
+
+.site-footer__links a:hover {
+  color: var(--pokemon-blue);
+}
+
 .page-stack {
   position: relative;
   display: grid;
@@ -3057,6 +3098,33 @@ button:disabled,
   font-weight: 850;
 }
 
+.legal-page__updated {
+  margin: 0;
+  color: var(--muted);
+  font-size: 14px;
+  font-weight: 850;
+}
+
+.legal-section__body {
+  color: var(--ink-soft);
+  line-height: 1.7;
+}
+
+.legal-section__body p {
+  margin: 0;
+}
+
+.legal-source-list {
+  display: grid;
+  gap: 8px;
+  margin: 4px 0 0;
+  padding-left: 22px;
+}
+
+.legal-source-list a {
+  word-break: break-word;
+}
+
 .edit-history-panel {
   display: grid;
   gap: 16px;
@@ -5659,6 +5727,12 @@ button:disabled,
     padding-bottom: 44px;
   }
 
+  .site-footer {
+    padding-right: 16px;
+    padding-left: 16px;
+    padding-bottom: 28px;
+  }
+
   .page-title {
     font-size: 32px;
   }