Add @trails-cool/api package with Zod schemas for REST API contract

Single source of truth for the Journal REST API:
- API_VERSION semver constant (1.0.0)
- Endpoint path constants (routes, activities, auth, uploads, push)
- Zod schemas for all request/response shapes:
  - Routes: list, detail, create, update, compute
  - Activities: list, detail, create
  - Auth: token exchange, refresh, device management
  - Discovery: instance info + API version
  - Uploads: presigned URL flow
  - Errors: structured error response with field-level details
- Cursor-based pagination schemas
- TypeScript types inferred from Zod (z.infer)

Used by Journal server (validation) and mobile client (type safety).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/api/src/errors.ts

@@ -0,0 +1,26 @@
+import { z } from "zod";
+
+export const FieldErrorSchema = z.object({
+  field: z.string(),
+  message: z.string(),
+});
+
+export const ApiErrorResponseSchema = z.object({
+  error: z.string(),
+  code: z.string(),
+  fields: z.array(FieldErrorSchema).optional(),
+});
+
+export type FieldError = z.infer<typeof FieldErrorSchema>;
+export type ApiErrorResponse = z.infer<typeof ApiErrorResponseSchema>;
+
+/** Standard error codes */
+export const ERROR_CODES = {
+  VALIDATION_ERROR: "VALIDATION_ERROR",
+  NOT_FOUND: "NOT_FOUND",
+  UNAUTHORIZED: "UNAUTHORIZED",
+  FORBIDDEN: "FORBIDDEN",
+  CONFLICT: "CONFLICT",
+  RATE_LIMITED: "RATE_LIMITED",
+  INTERNAL_ERROR: "INTERNAL_ERROR",
+} as const;