refactor: eliminate code duplication with shared utilities

Created 5 new utility modules in apps/server/src/lib/ to eliminate ~320 lines of duplicated code:
- image-handler.ts: Centralized image processing (MIME types, base64, content blocks)
- prompt-builder.ts: Standardized prompt building with image attachments
- model-resolver.ts: Model alias resolution and provider routing
- conversation-utils.ts: Conversation history processing for providers
- error-handler.ts: Error classification and user-friendly messages

Updated services and providers to use shared utilities:
- agent-service.ts: -51 lines (removed duplicate image handling, model logic)
- auto-mode-service.ts: -75 lines (removed MODEL_MAP, duplicate utilities)
- claude-provider.ts: -10 lines (uses conversation-utils)
- codex-provider.ts: -5 lines (uses conversation-utils)

Added comprehensive documentation:
- docs/server/utilities.md: Complete reference for all 9 lib utilities
- docs/server/providers.md: Provider architecture guide with examples

Benefits:
- Single source of truth for critical business logic
- Improved maintainability and testability
- Consistent behavior across services and providers
- Better documentation for future development

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/server/src/lib/error-handler.ts

@@ -0,0 +1,104 @@
+/**
+ * Error handling utilities for standardized error classification
+ *
+ * Provides utilities for:
+ * - Detecting abort/cancellation errors
+ * - Detecting authentication errors
+ * - Classifying errors by type
+ * - Generating user-friendly error messages
+ */
+
+/**
+ * Check if an error is an abort/cancellation error
+ *
+ * @param error - The error to check
+ * @returns True if the error is an abort error
+ */
+export function isAbortError(error: unknown): boolean {
+  return (
+    error instanceof Error &&
+    (error.name === "AbortError" || error.message.includes("abort"))
+  );
+}
+
+/**
+ * Check if an error is an authentication/API key error
+ *
+ * @param errorMessage - The error message to check
+ * @returns True if the error is authentication-related
+ */
+export function isAuthenticationError(errorMessage: string): boolean {
+  return (
+    errorMessage.includes("Authentication failed") ||
+    errorMessage.includes("Invalid API key") ||
+    errorMessage.includes("authentication_failed") ||
+    errorMessage.includes("Fix external API key")
+  );
+}
+
+/**
+ * Error type classification
+ */
+export type ErrorType = "authentication" | "abort" | "execution" | "unknown";
+
+/**
+ * Classified error information
+ */
+export interface ErrorInfo {
+  type: ErrorType;
+  message: string;
+  isAbort: boolean;
+  isAuth: boolean;
+  originalError: unknown;
+}
+
+/**
+ * Classify an error into a specific type
+ *
+ * @param error - The error to classify
+ * @returns Classified error information
+ */
+export function classifyError(error: unknown): ErrorInfo {
+  const message = error instanceof Error ? error.message : String(error || "Unknown error");
+  const isAbort = isAbortError(error);
+  const isAuth = isAuthenticationError(message);
+
+  let type: ErrorType;
+  if (isAuth) {
+    type = "authentication";
+  } else if (isAbort) {
+    type = "abort";
+  } else if (error instanceof Error) {
+    type = "execution";
+  } else {
+    type = "unknown";
+  }
+
+  return {
+    type,
+    message,
+    isAbort,
+    isAuth,
+    originalError: error,
+  };
+}
+
+/**
+ * Get a user-friendly error message
+ *
+ * @param error - The error to convert
+ * @returns User-friendly error message
+ */
+export function getUserFriendlyErrorMessage(error: unknown): string {
+  const info = classifyError(error);
+
+  if (info.isAbort) {
+    return "Operation was cancelled";
+  }
+
+  if (info.isAuth) {
+    return "Authentication failed. Please check your API key.";
+  }
+
+  return info.message;
+}