File-size decomposition (no behavior change). Move the self-contained
SimpleEventEmitter and extractWebSeedsFromMagnet (+ its WEBSEED_PARAM_KEYS and
the now-sole safeDecodeURIComponent consumer) out of playbackService.js into a
new js/services/playbackHelpers.js and import them back.
playbackService.js: 1350 -> 1274 lines (now under its original 1289 baseline).
Re-recorded the smaller baseline in check-file-size.mjs to lock the reduction.
Build + full unit suite green.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
