fix (config): deterministic JSON key ordering for config.json

[kisst]

Summary

• The connections field ([]map[string]any) in config was serialized using json.Marshal, which produces non-deterministic key ordering due to Go's random map iteration order
• This caused spurious diffs when config.json is kept under version control — keys would randomly reorder on every save even when no actual values changed
• Replaced json.Marshal with a custom marshaler that sorts map keys alphabetically before serialization, ensuring stable and deterministic output

Test plan

• Verify config saves produce identical output when no values have changed
• Verify config saves correctly reflect actual value changes
• Verify nested map values in connections are also sorted deterministically

[kisst]

@mickael-kerjean can you check this, I am spamming my filestash config git repo with a bunch of meaningless changes.

[mickael-kerjean]

I am head deep in some other things, I will give it a look when I have some time