Richer <ote-events> cards, a fields attribute, and a calendar layout

[hhkaos]

Summary

Follow-up to #28 (merged). While testing the MVP locally, a few gaps and one real bug came up:

• Bug fix: :host painted its own background (--ote-bg), so the widget showed an opaque box behind its content instead of blending into the host page — very visible in dark mode. :host is now transparent by default; only individual .event/.message surfaces keep a background.
• More visual fields: packages/preview-feed's PreviewEvent now carries image, price (cheapest priced offer), organizerName, tags, and attendanceMode — parsed from a JSON feed's image/offers/organizers, previously dropped entirely or buried in a generic details[] bag.
• A fields attribute so a site can choose exactly which of these render (fields="price,tags" — a full allow-list override, not a merge, over the default image,when,location,attendance,description).
• layout="calendar", lazy-loaded via its own esbuild entry point (dist/calendar-layout.js, never statically imported by main.ts) so it costs nothing for consumers who don't use it. Built on @event-calendar/core rather than the FullCalendar apps/preview already uses — FullCalendar injects its CSS into document.head, which never reaches into a Shadow DOM at all, while @event-calendar/core ships an importable stylesheet this widget captures and injects into its own shadow root. Confirmed end-to-end in Chrome: month grid renders, is fully styled, navigates, and events are clickable. Core+DayGrid comes out to ~44KB gzip, fetched only when layout="calendar" is actually requested (dist/ote-events.js itself only grew from ~7KB to ~11KB).
• Playground updated: a fields checkbox group (mirroring DEFAULT_FIELDS) and a calendar option in the layout select, both wired into the generated copy-paste snippet the same way show-past already was (omitted when it matches the default).

Test plan

• pnpm build && pnpm test && pnpm typecheck && pnpm lint — all green across the whole workspace
• Manual pass in Chrome against a synthetic feed covering every new field (image as both bare-string and {url,alt} form, multiple offers, zero-price offer, organizers, tags, all three attendance modes):
  • Cards and list layouts render image/price/tags/organizer/attendance correctly, respecting field-presence and the fields= override
  • Dark theme: confirmed the area behind/between cards is now transparent (page background shows through), not painted dark
  • A broken image URL disappears cleanly (no broken-image icon) — also unit-tested
  • layout="calendar": month grid renders styled inside the Shadow DOM, next/prev navigation works, clicking an event with no link is a safe no-op
• Clicking a calendar event with a link (opens in a new tab) wasn't manually re-verified this pass — same window.open(url, "_blank", "noopener") pattern already used elsewhere in the codebase, low risk

Closes no issue on its own (an enhancement pass on top of #27's MVP) — flagging back on #27 for visibility.

https://claude.ai/code/session_011dPq4KUvwFaPbSUmYXBGae