feat: Add automatic English and French localization

[raccommode]

Summary

• Add a client-side localization layer that follows each user's Jellyfin display language through the culture exposed on the document root.
• Provide French translations for SeerrFin-owned UI while keeping the existing English copy as the default and fallback.
• React to Jellyfin language changes and dynamically inserted SeerrFin UI without requiring a plugin-wide language setting.
• Localize configuration defaults, native grid controls, relative/status labels, and Letterboxd date formatting.
• Keep media titles, descriptions, cast names, and other user/content data untouched.

How I tested

• Ran dotnet build SeerrFin.sln -c Release with .NET 9: 0 warnings and 0 errors.
• Ran node --check against every injected JavaScript file.
• Used a local browser fixture that mirrors the relevant Jellyfin DOM behavior and verified:
  • fr-CA selects French automatically.
  • en-US selects English automatically.
  • Changing html.lang at runtime updates existing SeerrFin UI.
  • Dynamically inserted labels and accessibility attributes are translated.
  • Media titles and image accessibility labels remain unchanged.
• Confirmed the localization resource is embedded in the Release DLL.

An installable fork-only test build is available in release 1.6.6.2.

Environment

• Target: Jellyfin 10.11.x / ABI 10.11.0.0
• Build SDK: .NET 9
• Browser validation: local Chromium fixture on macOS
• Theme: default fixture styling

Pending real-instance validation

This PR is intentionally opened as a draft because the changes have not yet been verified on a real Jellyfin + Seerr installation. The fork-only package is ready for that server test; I will add the exact Jellyfin/browser environment and screenshots before marking the PR ready for review.