[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)

.github/workflows/test-ui.yaml

@@ -148,7 +148,7 @@ jobs:
 
       - name: Run Playwright tests (Shard ${{ matrix.shardIndex }}/${{ matrix.shardTotal }})
         id: playwright
-        run: npx playwright test --project=chromium --shard=${{ matrix.shardIndex }}/${{ matrix.shardTotal }} --reporter=blob
+        run: pnpm exec playwright test --project=chromium --shard=${{ matrix.shardIndex }}/${{ matrix.shardTotal }} --reporter=blob
         working-directory: ComfyUI_frontend
         env:
           PLAYWRIGHT_BLOB_OUTPUT_DIR: ../blob-report
@@ -230,7 +230,7 @@ jobs:
         run: |
           # Run tests with both HTML and JSON reporters
           PLAYWRIGHT_JSON_OUTPUT_NAME=playwright-report/report.json \
-          npx playwright test --project=${{ matrix.browser }} \
+          pnpm exec playwright test --project=${{ matrix.browser }} \
             --reporter=list \
             --reporter=html \
             --reporter=json
@@ -281,10 +281,10 @@ jobs:
       - name: Merge into HTML Report
         run: |
           # Generate HTML report
-          npx playwright merge-reports --reporter=html ./all-blob-reports
+          pnpm exec playwright merge-reports --reporter=html ./all-blob-reports
           # Generate JSON report separately with explicit output path
           PLAYWRIGHT_JSON_OUTPUT_NAME=playwright-report/report.json \
-          npx playwright merge-reports --reporter=json ./all-blob-reports
+          pnpm exec playwright merge-reports --reporter=json ./all-blob-reports
         working-directory: ComfyUI_frontend
 
       - name: Upload HTML report