fix(e2e): repair test bugs, trim browser matrix, add failure alert

[JesperDramsch]

Per the decision to fix + trim + keep non-blocking, this revives the Playwright E2E suite that had been red on every nightly run for ~6 months and silently ignored.

The failures were test bugs, not site bugs

1. Over-strict timezone assertion (countdown-timers.spec.js)

expect(timezone).toMatch(/^([A-Z][a-z]+\/[A-Z][a-z]+|UTC[+-]\d+)$/);

The template emits real IANA zones like America/Los_Angeles / America/New_York (underscores, multi-segment) and the UTC-12 AoE default — all rejected by that regex, so the test failed deterministically on real data. Replaced with Intl.DateTimeFormat-based validation that also accepts the UTC[±N] pseudo-zone. Unit-checked against real IANA zones, UTC offsets, and garbage.

2. Mobile multiselect click timeouts (conference-filters.spec.js)

bootstrap-multiselect option labels resolve but aren't "actionable" on the Pixel 5 viewport, timing out at 10s. Use click({ force: true }) for those 3 option clicks (the test.skip guards for missing options are unchanged).

Maintenance changes

• Trim CI matrix 5 → 2 browsers: chromium + mobile-chrome. firefox/webkit/mobile-safari produced most of the historical cross-browser flakiness for little signal on a JS-light Jekyll site. They remain available via workflow_dispatch and the local Playwright config.
• Add notify-failure job: opens/updates a tracking issue when an unattended run (nightly cron or push to main) fails — so a red suite can't rot unnoticed for months again.
• Stays non-blocking: no pull_request trigger; data/conference PRs are never gated on it.

Verification

• playwright test --list parses the config and all 5 specs cleanly (90 chromium tests).
• Timezone validation logic unit-tested.
• ⚠️ The full browser run requires the Jekyll server + browser binaries — best validated by triggering the workflow (workflow_dispatch) once merged, or via the next nightly.

Follow-up (separate, optional)

The specs are written defensively with many test.skip() guards, so a "pass" can mean "selector not found, skipped." Worth tightening assertions later if the suite proves valuable.