Add per-track selection checkboxes, select-all control and a selection count to download modals (missing/YouTube/Tidal/artist-album). Implement JS helpers (toggleAllTrackSelections, updateTrackSelectionCount) to manage checkbox state, row dimming, button disabling, and to filter/stamp selected tracks with _original_index before sending to the backend. Update start/add-to-wishlist flows to use only selectedTracks and disable controls once analysis starts. Backend _run_full_missing_tracks_process now reads _original_index to preserve original table indices in analysis results. CSS updates (mobile.css and style.css) add styling for checkbox columns, responsive hiding logic for headers/columns, selection visuals (.track-deselected), and small layout/width tweaks.
