feat: add essence infusion upgrades as a late-prestige essence sink

Adds five global income multiplier upgrades (Essence Infusion I–V) purchasable
purely with essence at escalating costs (1T → 500T essence). These give late-
prestige players a meaningful ongoing use for the massive essence reserves that
accumulate once all other upgrades are purchased.

apps/api/src/data/upgrades.ts

@@ -767,4 +767,70 @@ export const defaultUpgrades: Array<Upgrade> = [
     target:     "adventurer",
     unlocked:   false,
   },
+  // ── Essence Sinks ─────────────────────────────────────────────────────────
+  {
+    costCrystals: 0,
+    costEssence:  1e12,
+    costGold:     0,
+    description:
+      "Channel a vast reservoir of essence into the guild's core — all production ×2.",
+    id:         "essence_sink_1",
+    multiplier: 2,
+    name:       "Essence Infusion I",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
+  {
+    costCrystals: 0,
+    costEssence:  5e12,
+    costGold:     0,
+    description:
+      "A deeper infusion saturates every operation with raw essence — all production ×2.",
+    id:         "essence_sink_2",
+    multiplier: 2,
+    name:       "Essence Infusion II",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
+  {
+    costCrystals: 0,
+    costEssence:  2.5e13,
+    costGold:     0,
+    description:
+      "Essence floods the ley-lines binding your guild — all production ×2.",
+    id:         "essence_sink_3",
+    multiplier: 2,
+    name:       "Essence Infusion III",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
+  {
+    costCrystals: 0,
+    costEssence:  1e14,
+    costGold:     0,
+    description:
+      "The guild breathes essence as its very lifeblood — all production ×3.",
+    id:         "essence_sink_4",
+    multiplier: 3,
+    name:       "Essence Infusion IV",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
+  {
+    costCrystals: 0,
+    costEssence:  5e14,
+    costGold:     0,
+    description:
+      "Essence transcends material form and reshapes reality itself — all production ×5.",
+    id:         "essence_sink_5",
+    multiplier: 5,
+    name:       "Essence Infusion V",
+    purchased:  false,
+    target:     "global",
+    unlocked:   true,
+  },
 ];

apps/web/src/components/game/aboutPanel.tsx

@@ -34,7 +34,10 @@ const howToPlay = [
       + " for the current run and stack multiplicatively — two ×2 upgrades"
       + " targeting the same adventurer combine to give ×4, not ×3. Global"
       + " upgrades multiply on top of adventurer-specific ones, so stacking"
-      + " both types compounds the effect significantly.",
+      + " both types compounds the effect significantly. Late in a run, look"
+      + " for the Essence Infusion upgrades — five powerful global multipliers"
+      + " purchasable purely with essence, giving that resource an ongoing"
+      + " use when gold upgrades are all bought.",
     title: "🔧 Upgrades",
   },
   {