Add Code tab with JavaScript export

[eviltester]

Closes #11

[Copilot]

Pull request overview

This PR restructures the export format tabs by introducing a grouped Code tab (currently containing a JavaScript sub-tab) and adds Playwright-based browser smoke tests + CI wiring to catch runtime initialization errors.

Changes:

• Refactor TabbedTextControl to render tabs from definitions and support grouped subtasks (e.g., Code → JavaScript).
• Add Playwright smoke tests with a local http-server webServer config, plus CI steps to install Chromium and run the browser suite.
• Add styling for the new subtask UI and update repo hygiene files (.gitignore, Copilot instructions, etc.).

Reviewed changes

Copilot reviewed 9 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
js/gui_components/tabbed-text-control.js	Adds tab definitions and grouped “Code” tab with subtask rendering/activation logic.
tests/utils/tabbed-text-control-mode.test.js	Adds unit tests for Code tab/subtask behavior and default selection bootstrapping.
styles.css	Adds CSS for conversion subtasks and active “main type” styling.
tests/browser/page-smoke.spec.js	Introduces Playwright smoke tests to verify app/generator pages initialize without browser errors.
playwright.config.js	Adds Playwright configuration + local web server for browser tests.
package.json	Adds Playwright/http-server deps, browser test scripts, and excludes browser tests from Jest runs.
package-lock.json	Locks new dependencies for Playwright/http-server.
.github/workflows/node.js.yml	Runs Playwright install + browser smoke tests in CI.
.gitignore	Ignores Playwright report and test-results output directories.
docs-src/blog/2026-04-27-export-improvements.md	Adds a release blog post describing new export features (XML/SQL/JSONL).
.github/copilot-instructions.md	Adds project contribution/testing guidance for Copilot-assisted work.