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>
+  )
+}