test: replace deprecated settings assertion

[setchy]

Summary

Replace Vitest's deprecated toThrowError matcher with supported toThrow in useSettingsStore.test.ts, preserving the existing error-message assertion.

Addresses the sonarjs/deprecation finding from the one-off test-code scan. Only this test file changes; no production behavior, dependencies, or configuration changes.

Validation

• Focused settings-store suite: 18 tests passed.
• Full suite: 1,865 tests passed across 186 files.
• TypeScript, formatting/lint checks, and production build passed. The build emitted a non-fatal chunk-size warning.
• Type-aware SonarJS recommended-rule scan of the changed file: zero findings and zero parsing errors. This is a local ESLint-plugin result, not a claim about SonarCloud issue closure.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud