Issues with Fastest server detection

[stenya]

Steps to Reproduce:

1. Modify the "Fastest Server" configuration and retain only a subset of servers.
2. In Firewall settings, enable Always-on firewall and disable Allow access to IVPN servers.... This prevents latency (ping) measurements to servers.
3. Restart ivpn-service (or reboot the PC).
4. Select "Fastest server" and initiate a connection.

Actual Result:
Due to the absence of latency data, the IVPN Client is unable to determine the fastest available server. - expected
It connects to a random server (e.g., "Buenos Aires"), even though it was excluded from the Fastest Servers configuration (step 1) — NOT expected.

[stenya]

Fix: 6c0d21b

Fixed bugs in Fastest Server detection (in fallback logic when no fastest server is found).

Additionally:
Add timezone-offset longitude estimate as a new intermediate fallback.
When server latency and geolocation data are unavailable, use timezone information to approximate the user's current longitude and select the nearest server. This provides only a rough east–west estimate (as it ignores latitude entirely), but it is still significantly better than selecting an arbitrary server.