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.

css/game.css

@@ -327,6 +327,42 @@ h2, h3 {
     opacity: 0.4;
 }
 
+/* Upgrades header + Buy All button */
+.upgrades-header {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    margin-bottom: 10px;
+}
+
+.upgrades-header h3 {
+    margin: 0;
+}
+
+.buy-all-btn {
+    background: var(--accent-gold);
+    color: var(--bg-primary, #1a1035);
+    border: none;
+    border-radius: 6px;
+    padding: 6px 14px;
+    cursor: pointer;
+    font-weight: bold;
+    font-size: 0.8rem;
+    transition: all 0.2s ease;
+}
+
+.buy-all-btn:hover:not(:disabled) {
+    background: #f1c40f;
+    transform: scale(1.03);
+}
+
+.buy-all-btn:disabled {
+    background: var(--text-muted);
+    color: var(--bg-primary, #1a1035);
+    cursor: not-allowed;
+    opacity: 0.4;
+}
+
 /* Side Panel Tabs */
 .tabs {
     display: flex;