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/server.js

@@ -1256,7 +1256,7 @@ apiRouter.use('/license', licenseRoutes({
   asyncHandler: ctx.asyncHandler
 }));
 apiRouter.use('/recipes', recipesRoutes(ctx));
-apiRouter.use(themesRoutes());  // No dependencies - standalone route
+apiRouter.use(themesRoutes({ asyncHandler }));
 
 // Inline routes on the API router
 apiRouter.get('/health', (req, res) => {
@@ -1824,33 +1824,14 @@ app.get('/api/docs/spec', asyncHandler(async (req, res) => {
   }
 }, 'api-docs-spec'));
 
-// JSON 404 catch-all for unmatched API routes
-app.use('/api', (req, res) => {
-  res.status(404).json({ success: false, error: `Not found: ${req.method} ${req.path}` });
-});
+// Unified error handlers (order matters!)
+const { notFoundHandler, errorMiddleware } = require('./error-handler');
 
-// Global error handler for typed errors
-app.use((err, req, res, next) => {
-  if (err instanceof AppError) {
-    return res.status(err.statusCode).json({
-      success: false,
-      error: err.message,
-      code: err.code,
-      ...(err.details ? { details: err.details } : {})
-    });
-  }
-  if (err instanceof ValidationError) {
-    return res.status(err.statusCode || 400).json({
-      success: false,
-      error: err.message,
-      errors: err.errors || undefined
-    });
-  }
-  // Catch-all: never leak stack traces or internal paths
-  const status = err.status || err.statusCode || 500;
-  log.error('server', 'Unhandled error', { error: err.message, path: req.path, method: req.method });
-  res.status(status).json({ success: false, error: status === 413 ? 'Request payload too large' : 'An internal error occurred' });
-});
+// 404 handler for unmatched API routes
+app.use('/api', notFoundHandler);
+
+// Global error handler (MUST be last middleware)
+app.use(errorMiddleware);
 
 // Export app for testing
 module.exports = app;