feat: add account management, email verification, AI fixes, and user guides

- Profile settings, account transfer, delete/leave account flows
- Email verification with JWT tokens and Resend integration
- AI assistant/copilot fixes: markdown rendering, shared RAG helpers,
  token tracking, input refocus, model_validate usage
- User guides hub + detail pages with 13 topic guides
- Sidebar and top bar navigation for guides

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/security.py

@@ -70,6 +70,19 @@ def create_password_reset_token(user_id: str) -> str:
     return jwt.encode(to_encode, settings.SECRET_KEY, algorithm=settings.ALGORITHM)
 
 
+def create_email_verification_token(user_id: str) -> str:
+    """Create a JWT email verification token (24-hour expiry, unique JTI)."""
+    jti = str(uuid.uuid4())
+    expire = datetime.now(timezone.utc) + timedelta(hours=24)
+    to_encode = {
+        "sub": user_id,
+        "type": "email_verification",
+        "jti": jti,
+        "exp": expire,
+    }
+    return jwt.encode(to_encode, settings.SECRET_KEY, algorithm=settings.ALGORITHM)
+
+
 def generate_temp_password(length: int = 16) -> str:
     """Generate a temporary password with guaranteed complexity.