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.

js/ui.js

@@ -189,15 +189,52 @@ const UI = {
         for (const [id, upgrade] of Object.entries(UPGRADES)) {
             const card = document.getElementById(`upgrade-${id}`);
             if (!card) continue;
-            
+
             const purchased = Game.state.upgrades.includes(id);
             const costEl = card.querySelector('.upgrade-cost');
-            
+
             if (costEl && !purchased) {
                 const canAfford = Game.state.stardust >= upgrade.cost;
                 costEl.textContent = canAfford ? `${Game.formatNumber(upgrade.cost)} ✨` : `❌ ${Game.formatNumber(upgrade.cost)} ✨`;
             }
         }
+
+        // Toggle the "Buy All" button: show it once at least one unowned,
+        // requirement-met upgrade exists, and disable it when nothing is
+        // affordable. Its enabled state is refreshed live each tick.
+        this.updateBuyAllButton();
+    },
+
+    /**
+     * Show/hide and enable/disable the "Buy All upgrades" button based on
+     * whether any unowned upgrade (whose requirements are met) is currently
+     * affordable. Read of game state only; never mutates it.
+     */
+    updateBuyAllButton() {
+        const btn = document.getElementById('buy-all-upgrades');
+        if (!btn) return;
+
+        let affordableAny = false;
+        for (const [id, upgrade] of Object.entries(UPGRADES)) {
+            if (Game.state.upgrades.includes(id)) continue;
+            if (Game.state.stardust < upgrade.cost) continue;
+            // Requirement check mirrors Game.buyUpgrade / buyAllUpgrades.
+            if (upgrade.requirements) {
+                let met = true;
+                for (const [reqId, reqCount] of Object.entries(upgrade.requirements)) {
+                    if ((Game.state.buildings[reqId] || 0) < reqCount) {
+                        met = false;
+                        break;
+                    }
+                }
+                if (!met) continue;
+            }
+            affordableAny = true;
+            break;
+        }
+
+        btn.style.display = affordableAny ? '' : 'none';
+        btn.disabled = !affordableAny;
     },
 
     /**