The Extract "All" button lights up when only some stems are selected #658

Reported as part of #657.

What goes wrong

The All button in the Extract row claims every stem is selected when only some are. It is not limited to one path: it is wrong after any page load.

Reproduced against a real import, choosing vocals and drums only:

So the one place it is reliably right is the moment after you click a stem, which is also the only moment you do not need it to tell you anything.

Why

syncAllBtn lives inside wireAllButton in static/js/main.js and is called in exactly two situations: once while wiring, and on a click of a stem chip or of All itself. Nothing else can reach it.

Both of the paths that set the selection without a click therefore leave it stale:

• On load. The saved selection is restored asynchronously (stemSelectionReady in static/js/state.js), after the wiring has already run its one sync against the all-stems default. The chips get the restored value, All keeps the default.
• Opening a library track. applyStoredStemSelection in static/js/catalog.js sets aria-pressed on each .stem-choice[data-stem] and never touches #stemAllBtn.

Constraints for whoever fixes it

• The state is derived, not stored: All is on exactly when selectedStems.size === STEM_NAMES.length. It needs one owner that every path goes through, rather than a third caller bolted on, or the next path added will miss it the same way.
• The restore is async and the wiring is not, so a fix that only reorders the calls will work until the store is slow.
• Whatever refreshes the chips should refresh All in the same place. refreshStemChoiceVisuals is the obvious candidate, since both paths above already call it or do its job by hand.