feat(creator): overhaul Creator flow, editor duplication, and staging setup (#83)

This pull request completely overhauls the Creator editor flow, resolves the editor duplication race condition, aligns layout/styling themes in light and dark mode, and adds Docker staging setups.

### Key Changes
1. **Creator Flow Polish**: Redesigned the editor canvas to prevent double scrolling by delegating overflow to the editor canvas layer, updated styles to a premium aesthetic.
2. **Race Condition Prevention**: Resolved Crepe editor duplication when loading or switching chapters by tracking state via shared window maps (`window.editorCache`, `window.editorStates`) and checking `_lastInitializedEditorId` synchronously in Blazor.
3. **Theme Synchronization**: Integrated explicit theme initialization (`ThemeService.InitializeAsync()`) and anchored CSS isolation selectors to correctly sync with Light (Soft Sepia) and Deep Dark theme preferences.
4. **Staging Automation**: Created staging docker configurations with `--nexus-only` flag to allow iterative development without resetting PG/Neo4j database containers.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #83
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

docker-compose.stage.yml

@@ -30,6 +30,7 @@ services:
       - ASPNETCORE_ENVIRONMENT=Staging
       - ConnectionStrings__PostgresConnection=Host=db;Database=${POSTGRES_DB:-nexus_stage_db};Username=${POSTGRES_USER:-nexus_user_stage};Password=${POSTGRES_PASSWORD:?POSTGRES_PASSWORD is required}
       - ConnectionStrings__QdrantConnection=http://qdrant:6334
+      - Qdrant__ApiKey=${QDRANT_API_KEY:-}
       - ConnectionStrings__Neo4jConnection=bolt://neo4j:7687
       - Neo4j__Username=${NEO4J_USERNAME:-neo4j}
       - Neo4j__Password=${NEO4J_PASSWORD:?NEO4J_PASSWORD is required}