Add encounter difficulty indicator (5.5e XP budget)

Live 3-bar difficulty indicator in the top bar showing encounter
difficulty (Trivial/Low/Moderate/High) based on the 2024 5.5e XP
budget system. Automatically derived from PC levels and bestiary
creature CRs.

- Add optional level field (1-20) to PlayerCharacter
- Add CR-to-XP and XP Budget per Character lookup tables in domain
- Add calculateEncounterDifficulty pure function
- Add DifficultyIndicator component with color-coded bars and tooltip
- Add useDifficulty hook composing encounter, PC, and bestiary contexts
- Indicator hidden when no PCs with levels or no bestiary-linked monsters
- Level field in PC create/edit forms, persisted in storage

Closes #18

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

specs/005-player-characters/spec.md

@@ -226,7 +226,7 @@ Deleting a player character MUST NOT remove or modify any combatants currently i
 
 ### Key Entities
 
-- **PlayerCharacter**: A persistent, reusable character template with a unique `PlayerCharacterId` (branded string), required `name`, `ac` (number), `maxHp` (number), `color` (string from predefined set), and `icon` (string identifier from preset icon set).
+- **PlayerCharacter**: A persistent, reusable character template with a unique `PlayerCharacterId` (branded string), required `name`, `ac` (number), `maxHp` (number), `color` (string from predefined set), `icon` (string identifier from preset icon set), and optional `level` (integer 1-20, added by spec 008 for encounter difficulty calculation).
 - **PlayerCharacterStore** (port): Interface for loading, saving, and deleting player characters. Implemented as a browser storage adapter.
 
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