Pre-download probe: format, quality and audio-track picker

Consolidates #222, #769, #953, #426 (and the format-listing part of many older requests).

The recurring ask: before downloading, probe the URL (extract_info with download=False) and let the user pick — resolution/format with file sizes (#222), audio track/language for multi-audio videos (#953, #769), and toggles like embedding the cover (#426).

Design direction (aligned with the playlist Browse modal accepted in #1030): the Download button becomes a split button; a "Browse formats…" action probes the URL and opens a modal listing available formats/tracks (reusing the existing selection framework). The chosen format_ids are submitted as a per-download format override — the backend already supports per-download ytdl_options_overrides. No new persistent state.

This is community-contribution territory (see the feature request guidelines). If you want to work on it, comment here first so we can agree on the UI before you invest in a PR. A good first slice would be video formats only, single URL (no playlists).