Migrate package manager from npm to pnpm.

Changes

• Add packageManager: pnpm@9.15.9 to root package.json
• Replace package-lock.json with pnpm-lock.yaml
• Add pnpm-workspace.yaml for monorepo workspace configuration
• Remove workspaces field from root package.json (pnpm uses pnpm-workspace.yaml)
• Move overrides to pnpm.overrides format
• Remove .npmrc (legacy-peer-deps is npm-specific; pnpm ≥8 has auto-install-peers=true by default)
• Update all npm run references to pnpm run in scripts
• Replace npm-run-all usage with sequential pnpm run calls
• Replace cd packages/x && npm run with pnpm --filter in root scripts
• Replace npm run build --workspaces --if-present with pnpm -r run build
• Use workspace:* protocol for local package references (dx-template-gallery-data)
• Use node ./script.js instead of pnpm run for scripts with args in CI (testcafe runner, start-apps) to avoid pnpm's -- injection breaking arg parsing
• Update all 4 CI workflows (lint, test, deploy, test-dev-with-latest-dx)
• Update README.md and packages/vue/README.md
• Update .gitignore with pnpm-debug.log*

Phantom dependencies resolved

• devexpress-gantt — added to packages/angular and packages/react (was transitively available via devextreme in npm's flat layout)
• react-router — added to packages/react (was transitively available via react-router-dom)