Add unit test coverage for the frontend build-version script and four small service gaps

[aglinxinyuan]

Task Summary

Five small frontend gaps, none of them large enough to be worth an issue alone:

• build-version.js sits at 8 of 15 lines — the build script that stamps the version manifest. Its file-writing path and its failure handling are unexercised.
• user-config.service.ts (60/63) and code-editor.service.ts each have a guard whose second arm is never taken.
• agent-registration.component.html is at 7 of 10 branch arms.
• notebook-migration.service.ts has one unreached line.

Explicitly out of scope: the six stub-*.service.ts files hold 29 further missed lines but are test doubles, not production code, and one of them is being deleted by the open project-removal work.

Task Type

• Testing / QA

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Generated-by: Claude Code (Opus 5)