feat: major content expansion with essence and crystal sinks

- 6 zones (up from 3): Shadow Marshes, Volcanic Depths, Astral Void
- 18 bosses (up from 4): 3 per zone with zone-based sequential unlock
- 24 quests (up from 9): 3-4 per zone, reorganised by zone
- 15 adventurer tiers (up from 10): Shadow Assassin through Divine Champion
- 28 equipment pieces (up from 12): boss drops + purchasable with essence/crystals
- 24 upgrades (up from 13): crystal-cost upgrades + new adventurer upgrades
- 22 achievements (up from 14): new milestones for bosses, quests, gold, armies
- Purchasable equipment system: buy items directly with essence or crystals
- Crystal-cost upgrades: spend crystals on global and click power boosts
- Zone-based boss progression: defeating a zone's last boss unlocks the next zone

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

@@ -32,10 +32,25 @@ const bonusDescription = (item: Equipment): string => {
 
 interface EquipmentCardProps {
   item: Equipment;
+  gold: number;
+  essence: number;
+  crystals: number;
 }
 
-const EquipmentCard = ({ item }: EquipmentCardProps): React.JSX.Element => {
-  const { equipItem } = useGame();
+const costLabel = (cost: { gold: number; essence: number; crystals: number }): string => {
+  const parts: string[] = [];
+  if (cost.gold > 0) parts.push(`🪙 ${cost.gold.toLocaleString()}`);
+  if (cost.essence > 0) parts.push(`✨ ${cost.essence.toLocaleString()}`);
+  if (cost.crystals > 0) parts.push(`💎 ${cost.crystals.toLocaleString()}`);
+  return parts.join("  ");
+};
+
+const EquipmentCard = ({ item, gold, essence, crystals }: EquipmentCardProps): React.JSX.Element => {
+  const { equipItem, buyEquipment } = useGame();
+
+  const canAfford = item.cost
+    ? gold >= item.cost.gold && essence >= item.cost.essence && crystals >= item.cost.crystals
+    : false;
 
   return (
     <div className={`equipment-card rarity-${item.rarity} ${item.equipped ? "equipped" : ""} ${!item.owned ? "not-owned" : ""}`}>
@@ -47,9 +62,22 @@ const EquipmentCard = ({ item }: EquipmentCardProps): React.JSX.Element => {
         </div>
         <p className="equipment-description">{item.description}</p>
         <p className="equipment-bonus">{bonusDescription(item)}</p>
+        {!item.owned && item.cost && (
+          <p className="equipment-cost">{costLabel(item.cost)}</p>
+        )}
       </div>
       <div className="equipment-action">
-        {!item.owned && <span className="equipment-locked">🔒 Not yet obtained</span>}
+        {!item.owned && !item.cost && <span className="equipment-locked">🔒 Boss drop</span>}
+        {!item.owned && item.cost && (
+          <button
+            className="equip-button"
+            disabled={!canAfford}
+            onClick={() => { buyEquipment(item.id); }}
+            type="button"
+          >
+            {canAfford ? "Purchase" : "Can't afford"}
+          </button>
+        )}
         {item.owned && item.equipped && <span className="equipment-equipped-badge">✓ Equipped</span>}
         {item.owned && !item.equipped && (
           <button
@@ -104,7 +132,13 @@ export const EquipmentPanel = (): React.JSX.Element => {
             <h3 className="slot-heading">{SLOT_LABEL[slotType]}</h3>
             <div className="equipment-list">
               {items.map((item) => (
-                <EquipmentCard key={item.id} item={item} />
+                <EquipmentCard
+                  key={item.id}
+                  item={item}
+                  gold={state.resources.gold}
+                  essence={state.resources.essence}
+                  crystals={state.resources.crystals}
+                />
               ))}
               {items.length === 0 && (
                 <p className="empty-zone">No items to show in this slot.</p>