feat(I18n) 添加I18n相关功能，支持中文和英文

src/client/components/FloatingHeader.css

@@ -7,8 +7,8 @@
 
   display: flex;
   align-items: center;
-  gap: 3rem;
-  padding: 0.75rem 1.5rem 0.75rem 2rem;
+  justify-content: space-between;
+  padding: 1.25rem 3rem;
 
   background: var(--card-bg);
   backdrop-filter: var(--glass-blur);
@@ -18,8 +18,8 @@
 
   box-shadow: 0 4px 20px -2px rgba(0, 0, 0, 0.1);
   transition: opacity var(--transition-smooth);
-  width: max-content;
-  max-width: 90vw;
+  width: calc(100% - 2rem);
+  max-width: 1100px;
 }
 
 .floating-header.scrolled {
@@ -45,6 +45,12 @@
   filter: drop-shadow(0 0 8px var(--accent-glow));
 }
 
+.header-controls {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+}
+
 .floating-header .theme-switcher {
   font-size: 1.1rem;
   padding: 6px 10px;