Show country names consistently in filters and sightings

Closes #159

What changed

The country filter showed names ("Norway"), but sighting results showed codes ("NO"). Now every place that shows a country uses one shared formatter:

• Added formatCountry in frontend/src/utils/format-sighting.ts. It uses the existing countryNameFromCode in utils/countries.ts, which the submit form already uses.
• formatLocation now ends with the country name, for example "Oslo, OS, Norway" instead of "Oslo, OS, NO". This changes the results list, map labels, the detail page heading and the page title.
• SearchFilters no longer has its own Intl.DisplayNames helper. The dropdown and the active-filter chips use formatCountry.

Missing and unknown countries

Not included: countries filled in from coordinates by the backfill (#123) can't be marked as uncertain. The API has no field that says which ones were inferred, and adding one would change the database and GraphQL schema. That should be its own issue if we want it.

Tests

• New unit tests for formatCountry with known, unknown and missing codes, plus updated formatLocation tests.
• Updated component and e2e tests that expected the old code-based text ("San Marcos, TX, US", "Oslo, NO").
• npm run lint, npm run format:check, npm run test and npm run test:e2e all pass.

How to check

1. npm run dev and npm run dev:backend
2. Pick a country in the filter. The chip and every result should show the same name.
3. Open a sighting. The heading and the tab title should show the country name.