Add a map view (layout="map") to the embed component

[hhkaos]

Motivation

The embed (apps/embed, <ote-events>) currently offers list and calendar
layouts. A map view would help visitors explore in-person events
geographically, which is especially useful for people who don't know the
local geography well (e.g. "what's happening near me" browsing).

Proposed behavior

Add layout="map" alongside the existing list/calendar values.

Events with location.geo — plotted as pins. This is the only case that
gets an exact pin.

Events with location.venue but no location.geo — do not
geocode them client-side. We considered a live geocoder (Nominatim, as
apps/editor/src/ui/map.ts#geocodeVenue already does) and a bundled
offline city gazetteer, and rejected both for the embed:

• A live geocoding call per pageview/per visitor doesn't scale and conflicts
  with Nominatim's usage policy (rate limits, no bulk/automated client use).
• An offline gazetteer avoids the network problem but not the accuracy one:
  location.venue is a single free-text string with no structured
  city/country, so matching it against a city list is exactly where it's
  fragile — common city names collide worldwide (Valencia, Córdoba,
  Cambridge, San José, Toledo, Guadalajara...). A confidently wrong pin is
  worse than no pin, particularly for the "I don't know this area" use case
  this view is meant to serve.

Instead: these events are excluded from the map and surfaced honestly —
a plain on-page notice near the map (e.g. "N events aren't shown on the map
because they don't have coordinates") plus a console.warn for the
integrating developer, both pointing at how to fix the source data (the
geocoding picker already in apps/editor, where a human confirms the pin
before it's published in location.geo). No pin is invented.

Online-only events (no location.venue at all) — these aren't "missing
coordinates", they have no physical location to plot. Don't lump them into
the "missing coordinates" notice above. Give them their own treatment: a
separate section/badge/count (e.g. "N online events") rather than silently
dropping them or conflating them with unmappable in-person events.

Hybrid events (location.geo and onlineUrl) — get a pin, with the
popup/card also noting the online option.

Non-goals

• No client-side/live geocoding of any kind in the embed bundle.
• No exact-vs-approximate pin distinction — an event either has geo and
  gets an exact pin, or it doesn't and isn't plotted.
• No deep-linkable map center/zoom URL state for this iteration — this ships
  as one view among the existing layouts, not a standalone map component.

Implementation notes

• Follow the src/calendar-layout.ts pattern: a separate module, never
  statically imported by src/main.ts, loaded via import() only when a
  consumer requests layout="map", so the base ote-events.js bundle stays
  minimal (per apps/embed/CLAUDE.md).
• Leaflet + OSM tiles is the existing precedent for maps in this repo
  (apps/editor/src/ui/map.ts) — reuse that approach/attribution rather than
  introducing a new mapping dependency.
• This is a new public layout value → needs a SemVer minor bump in
  apps/embed/package.json, a CHANGELOG.md entry, and the rest of the
  release checklist in apps/embed/CLAUDE.md.
• Read apps/embed/CLAUDE.md before starting.

Possible follow-up (separate issue)

A non-blocking packages/validate warning for location.venue present
without location.geo would surface this at publish/CI time instead of only
in the embed at render time, following the existing "absent field = absent +
warning" convention. Not part of this issue's scope.