test: add Playwright + Vitest harness with first E2E and unit suites

[ivanmaierg]

Summary

Bootstraps an end-to-end testing harness for the MV3 extension using Playwright (real Chromium with the unpacked extension loaded), plus a small Vitest unit suite for pure scheduling logic. Adds a CI e2e job that runs after build.

• 6 E2E specs covering popup, banner injection + dismiss, banner Refresh button → real tab reload, and SW message-passing
• 12 unit assertions on decideRefresh / decideRemind
• New CI job downloads the existing dist artifact (no double build) and runs pnpm test:e2e --headless=new

Why now

There was zero test infrastructure. The recent banner font fix (bfc20cc) is exactly the kind of regression a screenshot test catches; this PR adds that net plus enough behavioral coverage to ship changes with confidence.

Architecture decisions

• Two-pipeline split. Playwright for everything that touches the browser; Vitest for pure logic only (no parallel test universe). Alarm-driven flows aren't E2E-testable because chrome.alarms minimum period is 1 minute.
• Helper extraction. decideRefresh and decideRemind moved from inline branches in service-worker.ts into src/background/scheduler.ts as pure functions. Makes the unit suite possible without mocking chrome.*.
• Extension ID at runtime. manifest.config.ts has no key field, so the fixture parses the ID from serviceWorker.url() after waiting on context.waitForEvent('serviceworker').
• github.com mocked via page.route. Chromium matches content_scripts by URL not response origin, so the content script injects normally on a stub HTML response.
• Snapshot strategy. Linux baseline is committed in the repo (generated via Playwright Docker image); CI verifies against it. macOS *-darwin.png variants are gitignored.

Bundled SW changes (heads-up)

a6d398a is labeled "extract scheduling helpers" but the diff also carries pre-existing in-progress work that was on the working tree:

• ensureAlarm() helper — alarm creation is now idempotent (checks chrome.alarms.get first)
• Alarm cadence changed from periodInMinutes: 0.5 to 1 (matches the chrome.alarms minimum)
• log(...) debug logging added throughout (tick, alarm fire, tab activation, blur, reload)

These are real product improvements, not testing scaffolding. I bundled them into the extraction commit because they were already in the working tree when SDD apply ran. Open to splitting into a separate feat(background): idempotent alarm + structured logs commit if reviewer prefers — say the word.

Test coverage

Area	Tests
Popup	renders heading + Enabled toggle; toggling persists to chrome.storage and shows Saved
Banner	injects on mocked github.com; Dismiss hides it; Refresh now click triggers real tab reload via SW handler (sentinel-based assertion)
Banner styling	toHaveScreenshot Linux baseline committed; macOS/Windows local-only
SW alarm	gh-refresh-tick registered after init
SW messages	prefs-updated ack; refresh-now from non-tab sender no-ops but acks
Scheduler unit	12 assertions on refresh/remind decision math

Known gaps (intentional, not blockers)

1. Alarm-driven tick() not exercised end-to-end — unit tests cover the decision math, message-passing test covers the wiring, but the integration "alarm fires → walk all tabs → reload the right ones" is unverified. Closing this needs tick() exported for direct invocation. Filed as follow-up.
2. Snapshot regen is manual — when banner styling changes, regenerate the canonical Linux baseline via the Docker recipe in tests/README.md. CI does not auto-update.
3. Notifications + webNavigation.onHistoryStateUpdated are untested. Lower priority.

Test plan

• CI build job passes (existing)
• CI e2e job runs and passes against the committed Linux baseline
• If banner styling changed and the snapshot diff fails: regenerate the Linux baseline via the Docker recipe in tests/README.md, commit the updated reminder-banner-chromium-linux.png
• pnpm typecheck && pnpm lint && pnpm build green
• pnpm test:unit 12/12
• pnpm test:e2e 7/7 locally on macOS

Files

New:

• playwright.config.ts, vitest.config.ts
• src/background/scheduler.ts
• tests/e2e/fixtures/extension.ts, tests/e2e/fixtures/github-stub.html
• tests/e2e/popup.spec.ts, tests/e2e/banner.spec.ts, tests/e2e/service-worker.spec.ts
• tests/unit/scheduler.spec.ts, tests/unit/setup.ts
• tests/README.md

Modified:

• src/background/service-worker.ts (helper imports + bundled improvements above)
• package.json (devDeps: @playwright/test, vitest; scripts: test, test:unit, test:e2e, test:e2e:ui)
• eslint.config.js (tests/ override for react-hooks/rules-of-hooks on Playwright fixtures)
• .github/workflows/ci.yml (new e2e job)
• .gitignore (test artifacts + OS-specific snapshots)
• CHANGELOG.md