Bugfix: Improve single-shot location fetching

[fhasse95]

This PR improves LocationProvider.fetchCurrentLocation() reliability and performance on both platforms.

On iOS, single-shot location requests are now performed from the main actor and use requestLocation() instead of starting continuous location updates. Recent cached locations are reused when available, avoiding unnecessary delays while still falling back to a fresh location request when the cached location is stale.

On Android, fetchCurrentLocation() similarly checks available last-known locations first and only uses them when they are sufficiently recent. Otherwise, it falls back to a fresh single-shot location request.

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• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.
  I used Codex to generate the initial version of the changes, then iteratively refined them myself and manually validated the behavior in my own app, which uses the user's current location.

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