Refactor App.tsx from god component to context-based architecture

Replace prop drilling with React context providers. App.tsx shrinks
from 427 lines to ~80 lines of pure layout. Components consume shared
state directly via 7 context providers instead of threading 50+ props.

Key changes:
- 7 context providers wrapping existing hooks (encounter, bestiary,
  player characters, side panel, theme, bulk import, initiative rolls)
- 2 coordinating hooks extracted from App.tsx (useInitiativeRolls,
  useAutoStatBlock)
- All 9 affected components refactored from prop-based to context-based
- 6 test files updated to use providers or context mocks
- Prop count enforcement script (max 8 per component interface)
- Constitution principle II-A added (context-based state flow)

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

apps/web/src/contexts/index.ts

@@ -0,0 +1,7 @@
+export { BestiaryProvider } from "./bestiary-context.js";
+export { BulkImportProvider } from "./bulk-import-context.js";
+export { EncounterProvider } from "./encounter-context.js";
+export { InitiativeRollsProvider } from "./initiative-rolls-context.js";
+export { PlayerCharactersProvider } from "./player-characters-context.js";
+export { SidePanelProvider } from "./side-panel-context.js";
+export { ThemeProvider } from "./theme-context.js";