fix: return an error from CheckForUpdate instead of conflating failure with no-update

[youngjinjung-linq]

Summary

Implements #206: CheckForUpdate() returned an empty string for both "you are on the latest version" and "the update check failed", so broken checks were silently swallowed.

• Signature changed to CheckForUpdate(currentVersion string) (string, error); HTTP/API/parse failures are wrapped as update check failed: ....
• The TUI update check (internal/app/app.go) logs the error through the structured logger and still proceeds with an empty version, so startup behavior is unchanged for users.
• Cache and dev-version fast paths keep returning ("", nil).

Testing

• go test ./... passes; new tests cover the error path (wrapped error, empty version) and the newer-version path via a seam for the GitHub API call.

Closes #206

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📥 Commits

Reviewing files that changed from the base of the PR and between 744b139 and 49cc048.

📒 Files selected for processing (3)

• internal/app/app.go
• internal/update/update.go
• internal/update/update_test.go

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[YoungJinJung]

Recreated as #255 from an in-repo branch under the maintainer account; this PR is superseded.