Add Pathfinder 2e game system mode

Implements PF2e as an alternative game system alongside D&D 5e/5.5e.
Settings modal "Game System" selector switches conditions, bestiary,
stat block layout, and initiative calculation between systems.

- Valued conditions with increment/decrement UX (Clumsy 2, Frightened 3)
- 2,502 PF2e creatures from bundled search index (77 sources)
- PF2e stat block: level, traits, Perception, Fort/Ref/Will, ability mods
- Perception-based initiative rolling
- System-scoped source cache (D&D and PF2e sources don't collide)
- Backwards-compatible condition rehydration (ConditionId[] → ConditionEntry[])
- Difficulty indicator hidden in PF2e mode (excluded from MVP)

Closes dostulata/initiative#19

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/use-rules-edition.ts

@@ -1,7 +1,8 @@
 import type { RulesEdition } from "@initiative/domain";
 import { useCallback, useSyncExternalStore } from "react";
 
-const STORAGE_KEY = "initiative:rules-edition";
+const STORAGE_KEY = "initiative:game-system";
+const OLD_STORAGE_KEY = "initiative:rules-edition";
 
 const listeners = new Set<() => void>();
 let currentEdition: RulesEdition = loadEdition();
@@ -9,7 +10,14 @@ let currentEdition: RulesEdition = loadEdition();
 function loadEdition(): RulesEdition {
 	try {
 		const raw = localStorage.getItem(STORAGE_KEY);
-		if (raw === "5e" || raw === "5.5e") return raw;
+		if (raw === "5e" || raw === "5.5e" || raw === "pf2e") return raw;
+		// Migrate from old key
+		const old = localStorage.getItem(OLD_STORAGE_KEY);
+		if (old === "5e" || old === "5.5e") {
+			localStorage.setItem(STORAGE_KEY, old);
+			localStorage.removeItem(OLD_STORAGE_KEY);
+			return old;
+		}
 	} catch {
 		// storage unavailable
 	}