Preserve user-authored workspace settings during regeneration

[CheckPickerUpper]

Summary

• merge RePortal workspace identity settings into the existing JSONC settings object
• preserve unrelated settings, nested color customizations, top-level fields, and comments
• add a regression test covering identity regeneration and replacement of owned title-bar keys

Closes #6

Verification

• cargo test --all-targets

[github-actions]

Visual recap — skipped

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Recap skipped for 28b6d58: VISUAL_RECAP_API_KEY not configured (openai-compatible backend).