feat: Add theme system, improve form styling, fix import paths and data deletion bug

[Copilot]

Addresses optimization requirements: form beautification, light/dark theme support, bug fixes, and improved code readability.

Theme System

Added global CSS variable-based theming with light/dark mode:

• /core/styles/theme.css - Color, spacing, typography tokens
• /core/styles/forms.css - Form component styling using theme variables
• Theme toggle button in header (toggleTheme exposed via component API)

// App.vue
const currentTheme = ref<'light' | 'dark'>('light');
const toggleTheme = () => {
  currentTheme.value = currentTheme.value === 'light' ? 'dark' : 'light';
  document.documentElement.setAttribute('data-theme', currentTheme.value);
};

Bug Fixes

Import path case sensitivity (13 files)

• preview → preView in App.vue
• Icons → icons for SVG imports
• arrow → Arrow, dropdown → Dropdown for components

Data deletion bug in docInfo/formView/index.vue:86

// Before: incorrectly deleted contact when license was empty
if (!license.name) {
  delete data.contact;  // ❌ Wrong property
}

// After:
if (!license.name) {
  delete formData.license;  // ✅ Correct
}

Code Quality

• getData → getFormData, saveData → saveFormData (clearer intent)
• Removed debug console.log statements

Screenshots

Light Theme

Dark Theme

Original prompt

对OpenAPI设计器进行优化：
1、美化表单样式，让布局更加精细、美观舒适；
2、定义全局样式，支持浅色和深色两种主题；
3、检查代码bug并修复；
4、优化方法、变量命名提高可读性。

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