fix: preserve defaulted settings on partial updates

[itzptk]

Summary

• Split app settings validation into shared field definitions so the full settings schema keeps defaults, while partial update parsing leaves omitted fields omitted.
• Added a regression test proving non-default defaulted settings survive a partial update that only changes checkIntervalMinutes.

Closes #25

Verification

• node_modules/.bin/tsx -e "import { appSettingsUpdateSchema } from './src/shared/contracts.ts'; console.log(JSON.stringify(appSettingsUpdateSchema.parse({ checkIntervalMinutes: 60 })))" → {"checkIntervalMinutes":60}
• npm run test:node -- settings-store → 5 tests passed
• ELECTRON_OVERRIDE_DIST_PATH=/tmp npm run test:node → 15 files / 117 tests passed
• npm run build → Angular production build and tsup Electron build succeeded

Notes

• Initial full npm run test:node without ELECTRON_OVERRIDE_DIST_PATH was blocked by the local npm install not writing node_modules/electron/path.txt; using the override avoids Electron binary resolution for unit tests that do not launch Electron.