Session 14: Buy All Upgrades (bulk-buy for upgrades) + empty-slot boost leak fix

- 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.

index.html

@@ -58,7 +58,10 @@
                 </div>
 
                 <div id="upgrades-tab" class="tab-content active">
-                    <h3>Upgrades</h3>
+                    <div class="upgrades-header">
+                        <h3>Upgrades</h3>
+                        <button id="buy-all-upgrades" class="buy-all-btn" style="display:none" title="Buy every upgrade you can currently afford">Buy All</button>
+                    </div>
                     <div id="upgrades-list"></div>
                 </div>