feat(seo-plugin): i18n support for UI components and fix favicon infinite loop (#299, #300)

[khuepm]

Description

This PR introduces two major enhancements and fixes for the seo-plugin extension, addressing both UI localization capability and a performance bug during search preview rendering.

Resolves #299
Resolves #300

Changes Made

1. Multi-language (i18n) Support

• Replaced hardcoded English string literals with the $t() translation method across all Vue components (tabs, labels, field titles, placeholders, hints, and error notices).
• Added core translation keys systematically using the seo_plugin. prefix.
• Created structured YAML language files en-US.yaml and vi-VN to handle these translation pairs.
• Injected mergeLocaleMessage() directly into the main Vue bundle via src/lang/index.ts to ensure translations work seamlessly within the Directus Extension bundle constraints.
• Transformed static Dropdown options in fields.ts (e.g. Change Frequency, Priority) into dynamic translated options on render.

2. SearchPreview Favicon Infinite Loop Fix

• Fixed an issue in SearchPreview.vue where a broken favicon URL caused an infinite window.location.origin loop query due to missing error boundaries.
• Added an @error event handler (handleFaviconError) to correctly capture image load failures.
• Implemented a fallback mechanism to hide the favicon gracefully (showFavicon = false) instead of continually fetching.

Verification

• Validated that the SearchPreview component successfully catches image errors without looping.
• Tested the seo-plugin interface actively changing between English and Vietnamese interface modes within a local Directus environment.
• Verified that a full npm run build succeeds correctly with the translations bundled within app.js and api.js.

Checklist

• Tested changes locally.
• Rebuilt dist/ elements.
• Code follows the extension's existing structure and coding standards.