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

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";