Add hreflang support and dynamic OG image generation

[shigahi]

Summary

This PR implements two new SEO features for Worknot:

Issue #35: Hreflang Support for Multilingual SEO

Adds hreflang link tags to support multilingual sites and improve international SEO.

Features:

• New I18nOptions interface with enabled flag and defaultLocale setting
• New PageAlternate interface for configuring alternate language versions per page
• Generates hreflang link tags in HeadRewriter:
  • Self-referencing hreflang for the current page
  • Alternate language versions from page metadata
  • x-default for language selector pages
• UI in Advanced Settings to enable i18n and set default locale
• Per-page alternate language configuration in Page SEO section (visible when i18n is enabled)

Generated HTML example:

<link rel="alternate" hreflang="en" href="https://example.com/about">
<link rel="alternate" hreflang="ja" href="https://example.com/ja/about">
<link rel="alternate" hreflang="x-default" href="https://example.com/about">

Issue #36: Dynamic OG Image Generation

Automatically generates Open Graph images from page titles when no custom OG image is specified.

Features:

• New OgImageGenerationOptions interface with customizable:
  • Background color
  • Text color
  • Font size
• SVG-based OG image generation at /og-image/[slug] endpoint
• Auto-injects og:image and twitter:image meta tags when no custom image is set
• Images cached for 1 week for performance
• UI in Advanced Settings to enable and customize OG image generation

Generated endpoint:

• GET /og-image/about returns an SVG image with the page title

Auto-injected meta tags:

<meta property="og:image" content="https://example.com/og-image/about">
<meta property="og:image:width" content="1200">
<meta property="og:image:height" content="630">
<meta name="twitter:image" content="https://example.com/og-image/about">

Testing

• Build passes (npm run build)
• No TypeScript errors

Related Issues

Closes #35
Closes #36