fix: align preview layout store key

[Dailin521]

Summary

This fixes the persisted preview layout key mismatch reported in #1458.

Root cause

The electron-store schema defined the layout setting as ui.previewlayout, while the renderer reads and writes ui.previewLayout. Fresh installs therefore missed the schema default for the key the app actually uses, and existing users could lose their saved layout after the schema key is corrected unless the legacy key is migrated.

Changes

• rename the schema key to ui.previewLayout so the default matches the renderer store access
• add a migration that copies any existing ui.previewlayout value into ui.previewLayout and removes the legacy key
• add a focused regression test for the migration path

Verification

• npx vitest run src/store/migrations.test.ts
• npx eslint src/store/index.ts src/store/migrations.ts src/store/migrations.test.ts
• npx tsc --noEmit

Fixes #1458

[CLAassistant]

All committers have signed the CLA.

[manojVivek]

@Dailin521 Looking great, thank you so much for the contribution!

[manojVivek]

@all-contributors Please add @Dailin521 for code.

[allcontributors]

@manojVivek

I've put up a pull request to add @Dailin521! 🎉