feat: add multi-language support (i18n) with Spanish/English

[hbeneke]

What kind of changes does this PR include?

• Changes with larger consequences (logic, library updates, etc.)

Description

This PR adds a complete internationalization (i18n) system to the theme without duplicating pages.

Key changes:

• New i18n system (src/i18n/) with centralized translations for English (default) and Spanish
• Language toggle dropdown in the header with a globe icon for easy language switching
• Client-side language persistence using localStorage
• Content filtering by language - Posts, notes, and tags pages now filter content based on the selected language
• Schema updates - Added lang field to post and note content schemas
• Flash prevention - CSS-based solution to prevent "flash of untranslated content" on page load
• Example Spanish content - 3 posts and 1 note in Spanish for demonstration

** Breaking changes:**

• Posts and notes now require a lang field (defaults to "en" if not specified)
• The posts, notes, and tags listing pages will only show content matching the user's selected language
• Users switching languages will see different content based on available translations

How it works:

1. Language preference is stored in localStorage (key: lang)
2. On page load, content is filtered client-side via data-lang attributes
3. UI elements use data-i18n attributes for translation

solves #42

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