feat: simplify login page UX and add i18n English support

- Redesign LoginPage with minimal professional style
  - Remove animated gradient backgrounds and floating orbs
  - Remove marketing claims (99% accuracy, enterprise-grade)
  - Center login form with clean card design
- Add multi-language support (zh-TW, en-US)
  - Create LanguageSwitcher component in sidebar
  - Add en-US.json translation file
  - Persist language preference in localStorage
- Remove unused top header bar with search
- Move language switcher to sidebar user section

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/LanguageSwitcher.tsx

@@ -0,0 +1,45 @@
+import { useTranslation } from 'react-i18next'
+import { Globe } from 'lucide-react'
+
+const languages = [
+  { code: 'zh-TW', label: '繁體中文' },
+  { code: 'en-US', label: 'English' },
+]
+
+export default function LanguageSwitcher() {
+  const { i18n } = useTranslation()
+
+  const handleLanguageChange = (langCode: string) => {
+    i18n.changeLanguage(langCode)
+    localStorage.setItem('tool-ocr-language', langCode)
+  }
+
+  const currentLang = languages.find((lang) => lang.code === i18n.language) || languages[0]
+
+  return (
+    <div className="relative group">
+      <button
+        className="w-full flex items-center gap-2 px-3 py-2 text-sm text-sidebar-foreground/70
+          hover:text-sidebar-foreground hover:bg-white/5 rounded-lg transition-colors"
+      >
+        <Globe className="w-5 h-5" />
+        <span>{currentLang.label}</span>
+      </button>
+
+      {/* Dropdown */}
+      <div className="absolute left-0 bottom-full mb-1 py-1 bg-card border border-border rounded-lg shadow-lg
+        opacity-0 invisible group-hover:opacity-100 group-hover:visible transition-all z-50 min-w-[140px]">
+        {languages.map((lang) => (
+          <button
+            key={lang.code}
+            onClick={() => handleLanguageChange(lang.code)}
+            className={`w-full px-3 py-2 text-sm text-left hover:bg-muted transition-colors
+              ${i18n.language === lang.code ? 'text-primary font-medium' : 'text-foreground'}`}
+          >
+            {lang.label}
+          </button>
+        ))}
+      </div>
+    </div>
+  )
+}

frontend/src/components/Layout.tsx

@@ -2,6 +2,7 @@ import { Outlet, NavLink, useNavigate } from 'react-router-dom'
 import { useTranslation } from 'react-i18next'
 import { useAuthStore } from '@/store/authStore'
 import { apiClientV2 } from '@/services/apiV2'
+import LanguageSwitcher from '@/components/LanguageSwitcher'
 import {
   Upload,
   Settings,
@@ -11,8 +12,6 @@ import {
   LogOut,
   LayoutDashboard,
   ChevronRight,
-  Bell,
-  Search,
   History,
   Shield
 } from 'lucide-react'
@@ -96,9 +95,9 @@ export default function Layout() {
         </nav>
 
         {/* User section */}
-        <div className="px-3 py-4 border-t border-border/20">
+        <div className="px-3 py-4 border-t border-border/20 space-y-2">
           {user && (
-            <div className="flex items-center gap-3 px-3 py-2 rounded-lg bg-white/5 mb-2">
+            <div className="flex items-center gap-3 px-3 py-2 rounded-lg bg-white/5">
               <div className="w-8 h-8 rounded-full bg-primary flex items-center justify-center text-white font-semibold text-sm flex-shrink-0">
                 {user.username.charAt(0).toUpperCase()}
               </div>
@@ -108,6 +107,9 @@ export default function Layout() {
               </div>
             </div>
           )}
+          <div className="px-3">
+            <LanguageSwitcher />
+          </div>
           <button
             onClick={handleLogout}
             className="w-full flex items-center gap-3 px-3 py-2 rounded-lg text-sidebar-foreground/70 hover:text-sidebar-foreground hover:bg-white/5 transition-colors text-sm"
@@ -119,37 +121,11 @@ export default function Layout() {
       </aside>
 
       {/* Main content area */}
-      <div className="flex-1 flex flex-col overflow-hidden">
-        {/* Top bar */}
-        <header className="bg-card border-b border-border px-6 py-4">
-          <div className="flex items-center justify-between">
-            {/* Search bar - placeholder for future use */}
-            <div className="flex-1 max-w-2xl">
-              <div className="relative">
-                <Search className="absolute left-3 top-1/2 transform -translate-y-1/2 w-5 h-5 text-muted-foreground" />
-                <input
-                  type="search"
-                  placeholder="搜尋檔案或功能..."
-                  className="w-full pl-10 pr-4 py-2 bg-background border border-border rounded-lg text-sm focus:outline-none focus:ring-2 focus:ring-primary/20 focus:border-primary"
-                />
-              </div>
-            </div>
-
-            {/* Notifications */}
-            <button className="ml-4 p-2 rounded-lg hover:bg-muted transition-colors relative">
-              <Bell className="w-5 h-5 text-muted-foreground" />
-              <span className="absolute top-1 right-1 w-2 h-2 bg-destructive rounded-full"></span>
-            </button>
-          </div>
-        </header>
-
-        {/* Page content */}
-        <main className="flex-1 overflow-y-auto bg-background p-6 scrollbar-thin">
-          <div className="max-w-7xl mx-auto">
-            <Outlet />
-          </div>
-        </main>
-      </div>
+      <main className="flex-1 overflow-y-auto bg-background p-6 scrollbar-thin">
+        <div className="max-w-7xl mx-auto">
+          <Outlet />
+        </div>
+      </main>
     </div>
   )
 }