feat: add equipment, achievements, and visual polish

- Equipment system: 12 items across weapon/armour/trinket slots with
  common/rare/epic/legendary rarities; starter commons auto-equipped,
  higher tiers drop from boss victories
- Achievement system: 15 milestones with typed conditions; checked
  each tick and crystal rewards applied automatically
- Achievement toast: slide-in notification, auto-dismisses after 4s
- Floating click text: +X gold floats on each manual click
- Expanded quests (9 total) and upgrades (12 total)
- Upgrade panel now shows locked upgrades so players can see their
  progression path
- formatNumber utility (K/M/B/T) used consistently across all panels
- Backfill logic for existing saves to add new content gracefully
- types package now emits .d.ts declarations

apps/web/src/utils/format.ts

@@ -0,0 +1,20 @@
+/**
+ * Formats a number with K/M/B/T suffixes for display.
+ * Numbers below 1000 show one decimal place.
+ */
+export const formatNumber = (value: number): string => {
+  if (!isFinite(value) || isNaN(value)) return "0";
+  if (value >= 1_000_000_000_000) {
+    return `${(value / 1_000_000_000_000).toFixed(2)}T`;
+  }
+  if (value >= 1_000_000_000) {
+    return `${(value / 1_000_000_000).toFixed(2)}B`;
+  }
+  if (value >= 1_000_000) {
+    return `${(value / 1_000_000).toFixed(2)}M`;
+  }
+  if (value >= 1_000) {
+    return `${(value / 1_000).toFixed(2)}K`;
+  }
+  return value.toFixed(1);
+};