fix(markdown): preserve code backgrounds across theme changes

[xuzijian2019]

Summary

Fix code-fence and Mermaid-source backgrounds being reset after a theme change.

Prism's vs pre style uses backgroundColor, while vscDarkPlus uses background. Combining either with the component's constant customStyle.background makes React's style diff remove/update a competing property. On main (6fcd7d4), light → dark clears the intended background to transparent; switching back applies Prism's white background rather than the app color mix. React reports the shorthand/longhand conflict.

• Let the component own the pre's backgroundColor for highlighted and streaming code.
• Copy the dark pre theme and remove its background shorthand, without mutating the imported Prism theme.
• Add a static-render regression and real browser theme round trips for ordinary code and Mermaid source.

Refs xuzijian2019/pi-agent-desktop#8

Verification

• npm test: 2,284 passed, 0 failed.
• tsc --noEmit: passed.
• npm run lint: passed.
• Full npm run test:e2e (dev mode): passed, including new light/dark round trips at 1280px and 390px.
• Additional isolated browser reproduction/fix checks at 1280×800, 390×844 and 390×480; after the fix the computed backgrounds equal the intended color mix and no style-conflict warning is emitted.

No product CSS/theme redesign, SDK policy changes or fork UI dependencies. No inference requests used in verification. Browser verification used Chromium on macOS, not real mobile hardware.