refactor: complete error handling standardization (issue #101)

Prestige Store:
- Convert doPrestige() to return Result<void> with specific error codes
  (INVALID_PRESTIGE_ID, PRESTIGE_MAX_LEVEL, INSUFFICIENT_INSIGHT)
- Convert startPactRitual() to return Result<void> with specific error codes
  (GUARDIAN_NOT_DEFEATED, PACT_ALREADY_SIGNED, PACT_SLOTS_FULL,
   INSUFFICIENT_MANA, RITUAL_IN_PROGRESS)

Combat Actions:
- Add try/catch wrapper inside processCombatTick with safe fallback defaults
- Add makeDefaultCombatTickResult helper for error recovery

LocalStorage Error Handling:
- Create safe-persist.ts utility wrapping localStorage with error handling
  (corrupted JSON, quota exceeded, unexpected failures)
- Update all 8 Zustand stores to use createSafeStorage() in persist middleware

UI Updates:
- Update GuardianPactsTab to use Result pattern for ritual error messages

Tests:
- Update store-actions-combat-prestige.test.ts for Result return types
- Update store-actions.test.ts ManaStore tests for Result pattern
- Remove duplicate Prestige/Discipline sections from store-actions.test.ts
- All files under 400 line limit

601 tests pass (3 pre-existing failures in spire-utils.test.ts)

docs/project-structure.txt

@@ -334,7 +334,9 @@ Mana-Loop/
 │       │   │   ├── index.ts
 │       │   │   ├── mana-utils.ts
 │       │   │   ├── pact-utils.ts
+│       │   │   ├── result.ts
 │       │   │   ├── room-utils.ts
+│       │   │   ├── safe-persist.ts
 │       │   │   └── spire-utils.ts
 │       │   ├── constants.ts
 │       │   ├── crafting-apply.ts