feat: Integrate HTML templates with FastAPI

- Created template_helpers.py for centralized template rendering
- Added ux_features.css for enhanced UX styling
- Implemented JavaScript modules for:
  - Keyboard shortcuts (Ctrl+K, Ctrl+R navigation)
  - User preferences persistence
  - Undo/redo functionality (Ctrl+Z/Ctrl+Y)
  - Mobile responsive features
  - Touch gesture support
  - Accessibility features (ARIA, focus management)
  - Screen reader support
  - Color contrast compliance (WCAG)
  - Multi-screen support
- Updated page_controller.py and error_controller.py to use template helpers
- Created comprehensive template integration tests
- All templates verified: index.html, login.html, setup.html, queue.html, error.html
- Maintained responsive layout and theme switching
- Updated instructions.md (removed completed task)
- Updated infrastructure.md with template integration details

src/server/utils/template_helpers.py

@@ -0,0 +1,96 @@
+"""
+Template integration utilities for FastAPI application.
+
+This module provides utilities for template rendering with common context
+and helper functions.
+"""
+from pathlib import Path
+from typing import Any, Dict, Optional
+
+from fastapi import Request
+from fastapi.templating import Jinja2Templates
+
+# Configure templates directory
+TEMPLATES_DIR = Path(__file__).parent.parent / "web" / "templates"
+templates = Jinja2Templates(directory=str(TEMPLATES_DIR))
+
+
+def get_base_context(
+    request: Request, title: str = "Aniworld"
+) -> Dict[str, Any]:
+    """
+    Get base context for all templates.
+    
+    Args:
+        request: FastAPI request object
+        title: Page title
+        
+    Returns:
+        Dictionary with base context variables
+    """
+    return {
+        "request": request,
+        "title": title,
+        "app_name": "Aniworld Download Manager",
+        "version": "1.0.0"
+    }
+
+
+def render_template(
+    template_name: str,
+    request: Request,
+    context: Optional[Dict[str, Any]] = None,
+    title: Optional[str] = None
+):
+    """
+    Render a template with base context.
+    
+    Args:
+        template_name: Name of the template file
+        request: FastAPI request object
+        context: Additional context variables
+        title: Page title (optional)
+        
+    Returns:
+        TemplateResponse object
+    """
+    base_context = get_base_context(
+        request,
+        title or template_name.replace('.html', '').replace('_', ' ').title()
+    )
+    
+    if context:
+        base_context.update(context)
+    
+    return templates.TemplateResponse(template_name, base_context)
+
+
+def validate_template_exists(template_name: str) -> bool:
+    """
+    Check if a template file exists.
+    
+    Args:
+        template_name: Name of the template file
+        
+    Returns:
+        True if template exists, False otherwise
+    """
+    template_path = TEMPLATES_DIR / template_name
+    return template_path.exists()
+
+
+def list_available_templates() -> list[str]:
+    """
+    Get list of all available template files.
+    
+    Returns:
+        List of template file names
+    """
+    if not TEMPLATES_DIR.exists():
+        return []
+    
+    return [
+        f.name
+        for f in TEMPLATES_DIR.glob("*.html")
+        if f.is_file()
+    ]