refactor: move fallback error templates to shared directory with improved naming

Reorganize error template structure to consolidate shared templates in a dedicated location.

Changes:
- Move templates/fallback/* to templates/shared/ with -fallback suffix
  - 404.html → 404-fallback.html
  - 500.html → 500-fallback.html
  - generic.html → generic-fallback.html
- Update error_renderer.py to use new template paths
  - Modified template selection logic in _find_template()
  - Updated documentation strings to reflect new paths
- Update error rendering documentation
  - HTML_ERROR_RENDERING_FLOW_DIAGRAM.md: Updated diagrams and examples
  - ERROR_RENDERING_DEVELOPER_DOCUMENTATION.md: Updated file structure and paths

Benefits:
- Clearer organization with all shared templates in shared/ directory
- Consistent naming convention (-fallback suffix) indicates purpose
- Aligns with existing cdn-fallback.html in same directory

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/development/error-rendering/ERROR_RENDERING_DEVELOPER_DOCUMENTATION.md

@@ -85,7 +85,7 @@ app/templates/
 ├── admin/errors/         # Admin error pages (Phase 1 - COMPLETE)
 ├── vendor/errors/        # Vendor error pages (Phase 2 - PENDING)
 ├── shop/errors/          # Shop error pages (Phase 3 - PENDING)
-└── fallback/             # Generic fallback pages (COMPLETE)
+└── shared/               # Shared fallback error pages (COMPLETE)
 ```
 
 ---
@@ -153,8 +153,8 @@ ErrorPageRenderer.render_error_page(
 
 1. `{context}/errors/{status_code}.html` (e.g., `admin/errors/404.html`)
 2. `{context}/errors/generic.html` (e.g., `admin/errors/generic.html`)
-3. `fallback/{status_code}.html` (e.g., `fallback/404.html`)
-4. `fallback/generic.html` (absolute fallback)
+3. `shared/{status_code}-fallback.html` (e.g., `shared/404-fallback.html`)
+4. `shared/generic-fallback.html` (absolute fallback)
 
 **Template Variables Provided:**
 
@@ -353,10 +353,11 @@ app/templates/
 │   └── errors/
 │       └── (same structure as admin)
 │
-└── fallback/
-    ├── 404.html
-    ├── 500.html
-    └── generic.html
+└── shared/
+    ├── 404-fallback.html
+    ├── 500-fallback.html
+    ├── generic-fallback.html
+    └── cdn-fallback.html
 ```
 
 ### Base Template Pattern