fix(garmin): support regional MapShare endpoints and warn on full URL input

[techtimo]

Summary

• garmin_feed_share_base() now handles both bare usernames (JohnDoe) and full MapShare page URLs (https://eur-share.inreach.garmin.com/JohnDoe), building the correct Feed/Share API endpoint in either case — fixes data fetching for European/regional Garmin accounts
• FeedModal shows a warning notice with the extracted username suggestion when the mapshare_address field contains a full URL, guiding users to correct their input
• Provider field description updated to clarify only the username is expected

Test plan

• Add a Garmin inReach feed with a bare username — verify it fetches from share.garmin.com/Feed/Share/{username}
• Add a feed with a full URL (e.g. https://eur-share.inreach.garmin.com/User) — verify the warning notice appears with the extracted username, and fetching hits the correct regional endpoint
• Edit an existing feed with a bare username — verify no warning is shown

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/spotmap-admin/components/FeedModal.jsx	50.00%	10 Missing ⚠️

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