CELDEV-1304 - Filebase-Replacement: implement new MediaLib based on VueFinder

[fpichler]

https://synjira.atlassian.net/browse/CELDEV-1304

Summary

Adds a new celements-admin-frontend Vue 3/Vite application for the admin UI, including a MediaLib view built on VueFinder and a tag management workflow for files.

Changes

• Adds the initial Vue 3 admin frontend setup with Vite, TypeScript, Pinia, Vue Router, i18n, PrimeVue, Tailwind, ESLint, Prettier, and Vitest configuration.
• Adds SPA bootstrap support plus an embeddable <cel-admin> custom element entry point.
• Implements a MediaLib view using VueFinder with /api/files as the backend driver.
• Adds tag sidebar and status bar components for:
  • filtering files by labels
  • assigning/removing labels on selected files
  • creating, renaming, and deleting labels when permitted
• Adds a Pinia tag store that integrates with /api/files/tags/* endpoints and keeps local optimistic state for tag updates.
• Adds OIDC/Keycloak authentication service plumbing and auth store integration.
• Adds environment configs for dev, int, and prod.
• Adds Docker/Caddy deployment files and project README.