feat(app): split appearance mode and theme palettes

[murongg]

Summary

• Separate appearance mode from saved light and dark editor palettes.
• Preserve legacy theme and custom CSS values by migrating them into split preferences.
• Add isolated light/dark custom CSS editors and complete locale strings for the new labels.

Why

• The titlebar mode toggle should switch appearance without overwriting the user's chosen light or dark palette.
• Separate custom CSS fields avoid one custom theme stylesheet leaking across both appearances.

Validation

• pnpm --filter @markra/shared build passed
• pnpm --filter @markra/app test passed: 84 files, 1212 tests
• pnpm --filter @markra/app build passed
• Theme i18n key scan passed for all locale files

Risk

• Theme settings storage now reads legacy keys as fallback and writes the new split keys going forward.
• App tests still print existing jsdom Window.scrollBy() not implemented warnings, but the test command exits successfully.

Closes #314

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview, Comment	Jun 17, 2026 2:09pm