fix(claude): avoid false live conflicts on provider switch

[hutiefang76]

Summary

• Avoid reporting normal Claude provider changes as live configuration conflicts during switch preview.
• Keep the previous provider snapshot as the merge base before backfilling live settings, then use that base when preparing Claude settings.json.
• Add regression coverage for both the expected switch case and a real local override that should still report a conflict.

Fixes #287

Tests

• cargo fmt --manifest-path src-tauri/Cargo.toml -- --check
• git diff --check
• cargo test --manifest-path src-tauri/Cargo.toml preview_switch_live_conflicts -- --nocapture
• cargo test --manifest-path src-tauri/Cargo.toml provider_switch -- --nocapture

[unive3sal]

LGTM