fix: honor build --mode dotenv files

[WilliamK112]

Summary

Closes #2393.

This adds Vite-style vinext build --mode <mode> support for build-time dotenv loading:

• parse --mode / --mode=<mode> in the shared CLI parser
• load .env.<mode>* through vinext's Next-compatible dotenv loader before Vite config/build setup
• pass the same mode into Vite config metadata loading, createBuilder, and the hybrid Pages Router server build
• allow custom dotenv modes such as staging/cdn while preserving the existing test-mode .env.local skip
• document the build flag in vinext build --help

Validation

• corepack pnpm test tests/cli-args.test.ts tests/dotenv.test.ts (75/75)
• corepack pnpm exec tsc --noEmit --pretty false --project packages/vinext/tsconfig.json
• corepack pnpm run check
• corepack pnpm --filter vinext build
• git diff --check origin/main...HEAD
• Manual smoke: built a temporary Pages fixture with .env, .env.production, and .env.staging, ran node packages/vinext/dist/cli.js build --mode staging, and verified every vite.config load saw mode: "staging" plus process.env.NEXT_PUBLIC_TARGET === "from-staging" before the build completed.

[chatgpt-codex-connector]

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