fix: show country code when city is unavailable in location display

[Boanerges1996]

Summary

• The location computed property in participantsCard.vue required both city and country_code to be non-null before rendering a location string
• ipstack (and AppEngine geo headers) do not always return a city — VPN exit nodes, corporate proxies, and certain ISP IP ranges are often only mapped to a country
• Participants in those cases showed "Location: -" even though valid country data was available
• Fix: fall back to country_code alone when city is null

Before: "Location: -" for any participant where city is unknown
After: "Location: DE" (or whichever country) when only country is known, "Location: Berlin, DE" when both are available

Test plan

• Join from a VPN or corporate network — confirm location now shows at least the country code
• Join from a regular connection with city-level data — confirm full City, CC format is unchanged
• Confirm that participants with no geo data at all still show "Location: -"

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