Add a global selection store backing a checkbox per track row and a
floating SelectionBar above PlayerBar with Play next / Add to queue /
Add to playlist / Like all / Clear.
- selection/store.svelte.ts: id Set + TrackRef Map + anchor index;
toggleOne, selectRange (shift+click), clearSelection. Singleton —
layout effect clears on pathname change.
- TrackRow: checkbox slot replaces the track number on hover; row
click toggles selection once any row is picked; shift+click extends
the range. Esc clears (takes priority over closing the queue
drawer).
- SelectionBar: floating pill above PlayerBar, mounted inside the
QueryClientProvider so the Like-all action can resolve.
- player.playNextMany: bulk variant of playNext for "Play next" on a
multi-track selection.
Covered by selection-store unit tests and three new TrackRow tests
(checkbox toggle, sticky select-mode row click, shift+click range).
