fix(config): preserve profile GEO resource URLs

[SingLinkNetwork]

What changed

• Preserve non-empty geox-url values supplied by a subscription/profile when the client has not explicitly overridden that resource.
• Keep an explicit client GEO URL higher priority.
• Safely fill unspecified GEO resources from the built-in defaults.

Root cause

Profile generation replaced every subscription geox-url entry with the client settings map. This silently redirected the initial automatic GEO update to the defaults. A partial settings map could also throw while generating the profile.

Validation

• Added regression coverage for profile URL preservation, explicit client override, partial settings fallback, and default behavior.
• flutter test — passed locally.
• flutter analyze --no-fatal-infos — 0 errors; 10 pre-existing deprecation infos in lib/widgets/theme.dart.
• Full cross-platform GitHub Actions verification is required before this issue is considered fixed.

Tracks #87; source report: chen08209/FlClash#2215.