Add option to control server icon updates (Always / Ask / Never)

[SpoilerRules]

Right now, server icons update automatically if the remote one changes. This can feel jarring or even suspicious if an icon suddenly looks different.

It would be nice to give users more control:

• Always: keep auto-updating (current behavior in 2.0.0)
• Ask: show a dialog comparing old vs new icon, with the server entry's name and address, so the user can decide
• Never: don’t update automatically

This improves trust (no surprise changes) and gives cautious users peace of mind, while still allowing others to keep automatic updates without interruption.