fix: recover invalid persisted settings

[itzptk]

Summary

• Recover invalid persisted settings by repairing bad fields back to defaults instead of throwing during startup reads.
• Preserve valid persisted values and existing missing-key legacy repair behavior.
• Persist the repaired settings payload after recovery.

Fixes #12

Verification

• RED before fix: npm run test:node -- electron/services/settings-store.test.ts -t "repairs invalid persisted setting values" failed with a ZodError for checkIntervalMinutes: 30 at SettingsStore.getSettings.
• GREEN focused: npm run test:node -- electron/services/settings-store.test.ts -t "repairs invalid persisted setting values" passed.
• npm run test:node -- electron/services/settings-store.test.ts passed: 5 tests / 1 file.
• npx vitest run --exclude electron/services/background-scheduler.test.ts passed: 110 tests / 14 files.
• npm run build passed: Angular production build plus Electron tsup build.

Local verification note

• npm run test:node was attempted but the local fresh npm ci install left node_modules/electron/path.txt missing because npm blocked Electron install scripts; the unrelated background-scheduler.test.ts suite could not import Electron. The targeted settings-store suite and the remaining Node/Electron suites passed as listed above.