feat: add client template (React admin + auth shell)

Generalized from the vet reference client:
- @aetoskia/react-auth + @aetoskia/react-openapi from npm.aetoskia.com
  (.npmrc), no local aliases
- generic Home (resource cards + live preview table), Header, Footer
- AppTheme/shared-theme with MUI v5 type shims
- VITE_API_BASE_URL / VITE_AUTH_BASE_URL env wiring
Verified: tsc --noEmit + vite build pass against built @aetoskia packages.

standards/src/templates/client/CHANGELOG.md

@@ -0,0 +1,15 @@
+# Changelog
+
+All notable changes to this project are documented here.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/)
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+<!--
+Keep [Unreleased] current as you land changes. On release, reconcile it against
+the change (git log / git diff since the last tag), add any missing user-facing
+entries, and get the user's approval before promoting it into a version heading.
+See standards: Versioning & Changelog (registry://release).
+-->
+
+## [Unreleased]