[chore] Replace npx with pnpx across the codebase (#5329)

## Summary
- Migrated all `npx` commands to `pnpx` to align with the pnpm ecosystem
- Updated all occurrences across the codebase for consistency

## Changes
- **Package.json scripts**: Updated test:browser, preinstall, and
collect-i18n scripts
- **GitHub Actions workflows**: Updated all workflow files that use npx
(test-ui, i18n, update-manager-types, etc.)
- **Documentation**: Updated browser_tests/README.md and
docs/extensions/development.md
- **Husky pre-commit hook**: Updated lint-staged and tsx commands
- **MCP configuration**: Updated .mcp.json to use pnpx

## Test Plan
- [ ] CI tests pass
- [ ] Local development commands work with pnpx
- [ ] GitHub Actions workflows execute successfully

🤖 Generated with Claude Code

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5329-chore-Replace-npx-with-pnpx-across-the-codebase-2646d73d36508127bd43d14b8364aeb1)
by [Unito](https://www.unito.io)

docs/extensions/development.md

@@ -110,7 +110,7 @@ pnpm build
 
 For faster iteration during development, use watch mode:
 ```bash
-npx vite build --watch
+pnpm exec vite build --watch
 ```
 
 Note: Watch mode provides faster rebuilds than full builds, but still no hot reload