♻️ refactor(spa): use __DEV__ define instead of process.env.NODE_ENV (#14696)

* ♻️ refactor(spa): use __DEV__ define instead of process.env.NODE_ENV

The Vite `__DEV__` define and its global type declaration are already
in place (plugins/vite/sharedRendererConfig.ts, src/types/global.d.ts).
Replace `process.env.NODE_ENV` checks across SPA-only files with the
`__DEV__` boolean so the bundler can statically eliminate dev-only
branches in production builds.

Server-side files (app/, server/, libs/next, libs/trpc, libs/better-auth,
envs, instrumentation) and modules that are also imported by Next.js
SSR pages (e.g. components/Loading/BrandTextLoading) are intentionally
left untouched to avoid runtime `__DEV__ is not defined` errors.

* fix(vitest): define __DEV__ and related constants for test environment

Vitest runs outside the Vite SPA build pipeline, so the __DEV__ define
injected by sharedRendererDefine was not available during tests. This
caused ReferenceError: __DEV__ is not defined in any test file that
transitively imports code using the __DEV__ constant.

Add a  block to vitest.config.mts that mirrors the SPA defines:
- __DEV__: true (test is not production)
- __CI__: mirrors process.env.CI
- __ELECTRON__/__MOBILE__: false (not testing platform-specific code)

* fix: replace missed isDevEnv reference with __DEV__ in AgentMockDevtools

vitest.config.mts

@@ -30,6 +30,12 @@ const alias = {
 };
 
 export default defineConfig({
+  define: {
+    '__CI__': process.env.CI === 'true' ? 'true' : 'false',
+    '__DEV__': process.env.NODE_ENV !== 'production' ? 'true' : 'false',
+    '__ELECTRON__': 'false',
+    '__MOBILE__': 'false',
+  },
   optimizeDeps: {
     exclude: ['crypto', 'util', 'tty'],
     include: ['@lobehub/tts'],