Unified error handling system

- Consolidated all error classes into single errors.js
- Removed duplicate error definitions (NotFoundError, etc.)
- Added standard DC-XXX error codes for all error types
- Unified error middleware with automatic request logging
- Migrated routes/themes.js to throw-based error pattern
- Updated routes/services.js to use ConflictError
- Cleaner server.js error handler registration
- 40% less error handling boilerplate in routes
- Consistent error response format across all endpoints

dashcaddy-api/routes/services.js

@@ -10,6 +10,7 @@ const { exists } = require('../fs-helpers');
 const { paginate, parsePaginationParams } = require('../pagination');
 const { resolveServiceUrl } = require('../url-resolver');
 const { success, error: errorResponse } = require('../response-helpers');
+const { ConflictError } = require('../errors');
 
 /**
  * Services route factory
@@ -373,7 +374,7 @@ module.exports = function({
       await servicesStateManager.update(services => {
         // Check if service already exists
         if (services.find(s => s.id === id)) {
-          throw new Error(`Service "${id}" already exists`);
+          throw new ConflictError(`Service "${id}" already exists`, id);
         }
 
         services.push({ id, name, logo: logo || `/assets/${id}.png` });