New loading animation, improved server switch

[naltatis]

Switching from a working server to an unreachable one showed a white screen until WKWebView gave up (DNS error or ~60 s request timeout), because the connection state survived the server switch. Also swaps the loading indicator for a simpler one.

• Connection state resets when the active server changes, so the loading overlay appears right away for unreachable servers; switching between working servers still stays flicker-free thanks to the existing 2 s show delay.
• Loading spinner is now the "ring-resize" loader from svg-spinners (MIT), ported to reanimated; static under Detox as before.
• Server switch sheet closes immediately instead of after an artificial 500 ms delay.