Map a country's cities, linked to the city list

[engleek]

Part of #258. Stacked on #256 (merge that first). The "All studios in Rennes" links need #259's city filter, and the "Explore on the map" link needs its /places?country=, so merge #259 before this too.

Reworked: this PR first had a regional choropleth. It weighted big, nearly empty regions over the cities where the scene actually is, and it repeated the list. This is variant A from the prototype (branch chore/prototype-country-city-map), which you picked.

What it does

• One view, linked both ways: the cities section is the ranked list with the studio/association split, next to a map with a circle per city, sized by its studios and associations. Hovering or focusing a city highlights it in both.
• Choosing a city (click, Enter or Space, in the list or on the map) opens who's there: its studios and associations, each linking to its page, plus "All studios in Rennes" and "All associations in Rennes".
• "Explore France on the map" opens /places?country=FR. The full exploration stays there.

How

• Positions: a city sits at the mean of its organisations' coordinates (citiesOf, which replaces topCities and also returns each city's organisations for the drill-down).
• Outline: Natural Earth's public-domain admin-1 data, dissolved into one small TopoJSON outline per country by scripts/build-country-maps.mjs (240 countries, 496 KB in all, 3–15 KB each, with a pinned mapshaper run through npx). It's drawn in the browser, so d3 (d3-geo, topojson-client, d3-composite-projections) stays out of the server bundle.
• Fitting:
  • The view fits the mainland (the largest landmass and those within 12° of it) plus any city beyond it. French Guiana doesn't shrink France, Svalbard doesn't shrink Norway, and a city out there is still shown.
  • France, Spain and Portugal get inset boxes only when a city is overseas, so there are no empty boxes.
  • The US uses Albers USA.
  • Countries crossing the antimeridian are centred on themselves.

Tests

• Unit:
  • mainland France is fitted with Paris in the middle third, Rennes west and Marseille south;
  • an overseas city brings the insets, and it's placed inside them;
  • Japan fits its main islands, an outlying city (Longyearbyen) stays in view, and Russia is centred;
  • cities without coordinates are left out;
  • all 240 maps draw;
  • citiesOf's positions and lists.
• e2e: the seeded Antarctic city appears as a circle labelled "2 studios, 1 association". Choosing it in the list opens the drill-down: its "All studios in…" link goes to /studios?country=AQ&city=…, and the studio links to its page. "Explore Antarctica on the map" points to /places?country=AQ.
• build ✅, vitest 149/149 ✅, Playwright 74/74 ✅
• Checked with France on a local copy of production data, at desktop and phone widths, in light and dark mode.

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