Root cause: SaveSystemSettings calls updateEnvFile which rewrites .env on
any setting change, triggering the config watcher. The watcher sees API_TOKEN
in .env and replaces all UI-created tokens with "Environment token" records,
wiping out host-agent scoped tokens.
Fix: updateEnvFile now compares the new content with existing content and
skips the write if nothing changed. Since dark mode (and other UI settings)
are stored in system.json, not .env, toggling theme no longer triggers
unnecessary .env rewrites.
This prevents the config watcher from being triggered unnecessarily and
preserves UI-created API tokens when changing cosmetic settings.
Future improvement: Deprecate API_TOKEN/API_TOKENS from .env entirely and
make api_tokens.json the single source of truth (requires migration logic).
5d99fc2f2d