Add automated playwright tests

[edmcman]

Almost all changes are new tests.

The exceptions are:

1. Small changes to CI to add the new tests
2. The shadow DOM is left open in the development build to enable the new tests

[Copilot]

Pull request overview

This PR adds comprehensive end-to-end testing infrastructure using Playwright to test the LinkHints browser extension. The tests verify the tutorial workflow by simulating user interactions and checking that hints appear correctly and navigation works as expected.

Key changes:

• Adds automated Playwright test suite for the tutorial workflow
• Modifies shadow DOM mode to be "open" in development builds to enable test access
• Updates CI/CD pipeline to run tests on all branches with a dedicated test job

Reviewed changes

Copilot reviewed 18 out of 29 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/tutorial.spec.ts	New comprehensive e2e test file covering the tutorial workflow with hint activation, navigation, and interactions
tests/-snapshots/.html	HTML snapshot files for visual regression testing of shadow DOM content
tests/-snapshots/.png	Screenshot snapshots for visual regression testing
playwright.config.ts	Playwright configuration with Firefox project, timeouts, and snapshot settings
src/renderer/Program.ts	Changes shadow DOM mode from "closed" to "open" in development for test access
package.json	Adds Playwright dependencies and test:playwright script
package-lock.json	Lock file updates for new Playwright dependencies
.github/workflows/ci.yml	Restructures CI with separate deploy, check, and test jobs; adds Playwright test execution
CONTRIBUTING.md	Documents the new automated testing workflow
.prettierignore	Excludes test results and snapshot directories from formatting
.gitignore	Excludes test results and tool version files
.github/copilot-instructions.md	New comprehensive project documentation for AI assistants
.devcontainer/devcontainer.json	New devcontainer configuration with Playwright setup

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