Reorganize connector registry management for better separation of concerns:

Build-time improvements:
- Create fetch-registry.js: Centralized utility for fetching and caching registry
  - Handles cache check, network fetch fallback, and minimal data extraction
  - Reusable by all build scripts without duplication
  - Reduced console logging for cleaner build output
- Rename prepare-sidebar-data.js to prepare-registry-cache.js
  - More accurately reflects its purpose as a registry cache preparer
  - Updated package.json scripts to use new name
- Consolidate constants into src/scripts/constants.js
  - Centralized registry paths, URLs, and doc directory constants
  - Used by all scripts: fetch-registry, prepare-registry-cache, cleanup-cache
- Simplify connector_registry.js to original approach (network fetch only)
  - No complex caching logic (handled by fetch-registry now)
  - Maintains original compatibility with remark plugins

Client-side improvements:
- Create clientRegistryUtils.js: Browser-safe registry lookup
  - Uses fetch to retrieve registry at runtime
  - Separated from build-time utilities to prevent webpack bundling issues
- Update RequestERD and EntityRelationshipDiagram components
  - Use clientRegistryUtils instead of build-time registry
  - Components now self-contained and independent of build process

Webpack fixes:
- No longer attempt to bundle Node.js code (fs, path modules)
- Remark plugins now use fetch-registry (async, build-time only)
- Client components use clientRegistryUtils (browser-safe)

Benefits:
✓ Clean separation: Build-time vs Runtime vs Client-side code
✓ Single responsibility: Each script has one clear purpose
✓ DRY principle: No duplicate constants or logic
✓ Reduced logging: Cleaner build output
✓ Better maintainability: Easier to understand data flow