Feat: add public embed page.

[arabcoders]

This pull request refactors the backend embed preview logic, introduces a new static embed endpoint, improves multimedia preview rendering for users, and enhances the frontend admin tokens table with better mobile support and loading states. It also includes minor fixes and new tests for embed previews.

Backend: Embed Preview Refactor and Static Embed Endpoint

• Extracted embed preview logic into a new module (embed_preview.py) for better reusability, including bot detection and rendering functions.
• Added a new /e/{token} endpoint (token_embed) that renders a static multimedia preview for a shared token, always showing the preview to users (not just bots).
• Refactored /f/{token} to delegate bot detection and preview rendering to the new module, simplifying the route and ensuring consistent behavior. [1] [2]

Frontend: Admin Tokens Table Improvements

• Redesigned the AdminTokensTable.vue component to support a responsive, card-based mobile layout and added loading/empty states for better UX. [1] [2] [3]

Embed Preview Rendering Enhancements

• Updated share_preview.html to show fullscreen video/audio previews for user requests on the new static embed endpoint, improving the viewing experience. [1] [2] [3]

Testing and Configuration

• Added a test for the new /e/{token} endpoint to ensure it renders the correct preview for public tokens.
• Set FBC_ALLOW_PUBLIC_DOWNLOADS=0 in test config to avoid side effects in test environments.

Other Fixes

• Fixed temporary file suffix and enforced output format in ensure_faststart_mp4 utility for more reliable MP4 processing. [1] [2]