Fix community Leading Teachers country filter and avatar placeholders

[bernardhanna]

Summary

• Filter Community Leading Teachers by the selected country_iso (same pattern as ambassadors), so country pages like Lithuania only show that country's teachers.
• Always show an avatar on the map sidebar / ambassador cards, using local images/default.png when the stored avatar is empty, a known default, or fails to load from S3.
• Encode teacher fields with @json to avoid JS breakage from special characters in names/paths.

Test plan

• Open /community?country_iso=LT and confirm only Lithuanian Leading Teachers appear in the map sidebar (~16), with markers for their cities.
• Confirm teachers without a real avatar show the local placeholder instead of a broken image.
• Spot-check another country (e.g. FR/IT) still filters correctly.
• php artisan test --filter=UserCommunityAvatarUrlTest

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