Keep save panes visible and persist theme choice

[woahitsraj]

Summary

• Keep the original save visibly rendered when a second save opens beside it in the Android Boxes view.
• Persist explicit theme choices and restore them before hydration.
• Use prefers-color-scheme when the user has not chosen a theme.
• Add Android and Playwright regression coverage for both behaviors.

Testing

• pnpm typecheck
• pnpm lint
• pnpm test:unit -- --run, 454 tests passed
• PKSX_E2E_PORT=4175 pnpm exec playwright test --project=chromium src/routes/boxes-two-save.e2e.ts
• PKSX_E2E_PORT=4178 pnpm exec playwright test --project=chromium src/routes/settings.e2e.ts, 9 tests passed
• PKSX_E2E_PORT=4177 pnpm exec playwright test --project=chromium src/routes/responsive-contract.e2e.ts --grep "large caps and representative surfaces"
• API 36 ControllerNavigationTest#openingSecondSaveKeepsOriginalPanePainted

Closes #178

Acceptance criteria

• Opening a second save side by side on Android leaves the original save visibly rendered and selectable.
• An explicit theme choice persists after the document or native WebView is recreated.
• With no explicit choice, the app uses prefers-color-scheme and resolves the theme before first paint.

Evidence

Android-width side-by-side saves

How verified: imported real Emerald and Pokémon X saves at 640 × 360, opened Emerald in Boxes, then opened X beside it. Emerald remains visibly rendered in the first pane.

android-two-save-flow.mp4

The API 36 instrumentation test also samples the rendered pixels in the original pane after the second save opens.

Persisted theme override

How verified: selected Dark in Settings, reloaded the document to model native WebView recreation, and observed Settings render dark immediately after reload.

dark-theme-persists.mp4

The Settings Playwright coverage additionally verifies the system dark preference when no override exists, then verifies that an explicit light override wins after reload.

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Updated for commit 6b3e1a0. View workflow run.