Add theme switching with light, dark, and auto modes

[Copilot]

Summary

This PR adds theme switching support to the Patch Reader browser extension (UI优化 - 支持主题切换，默认跟浏览器主题设置一致).

Features

• Theme Toggle UI: Added light/dark/auto toggle buttons in the header with emoji icons (☀️/🌙/💻) and text labels
• CSS Variables: Converted all hardcoded colors to CSS variables for consistent theming
• Dark Theme: Full dark theme support including diff2html component styling
• Auto Theme Detection: The "Auto" mode follows the browser's prefers-color-scheme setting (default)
• Theme Persistence: User's theme preference is saved to localStorage
• i18n Support: Added translations for theme labels in English, Chinese (浅色/深色/自动), and Japanese (ライト/ダーク/自動)
• Accessibility: Added ARIA attributes for screen reader support
• Flash Prevention: Early theme detection script prevents flash of wrong theme on page load

Screenshots

Light Theme

Dark Theme

Dark Theme with Diff Content

Checklist

• Add theme switching UI (light/dark/auto toggle buttons) in header
• Create CSS variables and dark theme styles
• Add JavaScript logic to detect browser theme preference (prefers-color-scheme)
• Add JavaScript logic to toggle themes and persist user preference
• Add i18n translations for theme labels in all locales (en, zh, ja)
• Build and manually verify the changes work correctly
• Take screenshots of UI changes
• Run code review and address feedback
• Run CodeQL security check (no vulnerabilities found)
• Bump version to 1.0.2

Original prompt

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This section details on the original issue you should resolve

<issue_title>UI优化</issue_title>
<issue_description>支持主题切换，默认跟浏览器主题设置一致</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes UI优化 #14

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[unknowIfGuestInDream]

@copilot 版本升级到1.0.2

[Copilot]

@copilot 版本升级到1.0.2

Done in 86748d9. Updated version to 1.0.2 in package.json, src/chrome/manifest.json, and src/edge/manifest.json.