Add combatant side assignment for encounter difficulty

Combatants can now be assigned to party or enemy side via a toggle
in the difficulty breakdown panel. Party-side NPCs subtract their XP
from the encounter total, letting allied NPCs reduce difficulty.
PCs default to party, non-PCs to enemy — users who don't use sides
see no change. Side persists across reload and export/import.

Closes #22

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

CLAUDE.md

@@ -69,6 +69,7 @@ docs/agents/               RPI skill artifacts (research reports, plans)
 - **oxlint** for type-aware linting that Biome can't do. Configured in `.oxlintrc.json`.
 - **TypeScript strict mode** with `verbatimModuleSyntax`. Use `.js` extensions in relative imports.
 - **Branded types** for identity values (e.g., `CombatantId`). Prefer immutability/`readonly` where practical.
+- **Reuse UI primitives** — before creating custom interactive elements (buttons, inputs, selects, dialogs), check `apps/web/src/components/ui/` for existing components with established variants and hover styles.
 - **Domain events** are plain data objects with a `type` discriminant — no classes.
 - **Tests** live in `__tests__/` directories adjacent to source. See the Testing section below for conventions on mocking, assertions, and per-layer approach.
 - **Quality gates** are enforced at pre-commit via Lefthook (parallel jobs). No gate may exist only as a CI step or manual process.