Core state (queue, index, state, position, duration, volume, shuffle,
repeat, error) plus actions: playQueue, togglePlay, linear skipNext,
skipPrev 3-second rule, seekTo, setVolume (localStorage-persisted),
registerAudioEl, reportTimeUpdate, reportDuration. Shuffle/repeat
logic and audio-event reporting land in follow-up commits.
Also install an in-memory Storage polyfill in vitest.setup.ts: Node 25
exposes a partial localStorage global that lacks getItem/setItem/clear
(requires --localstorage-file with a path), and jsdom defers to the
Node global when present, leaving tests unable to exercise storage.
Wires @tanstack/svelte-query for the data layer and
@testing-library/{svelte,jest-dom} for component-level tests. Registers
the jest-dom matchers via a new vitest setup file.
bvandeusen