- Added Game.buyAllUpgrades(): buys every affordable upgrade at once,
respecting per-upgrade requirements and looping until nothing remains.
- Added a 'Buy All' button to the Upgrades tab header; UI.updateBuyAllButton()
shows/hides & enables/disables it live each tick based on affordability.
- Fixed empty-slot branch of switchSlot(): added productionBoostUntil: 0 so a
temporary golden-bloom boost cannot leak into a fresh slot.
- Added .upgrades-header / .buy-all-btn CSS (gold accent theme).
Verified: headless test buy-all-upgrades-test.mjs 24/24 pass; existing
golden-bloom (22/22) and buy-max (6/6) tests re-run clean, no regressions.