fix: use Harness theme aliases in dark mode

[haotian-lu-prog]

Summary

• derive Side Chat surfaces, borders, text, controls, and bubbles from the Harness semantic theme aliases
• keep the legacy --dsw-* variables as fallbacks for compatibility with older hosts
• add regression coverage for the selection toolbar and annotation editor theme contracts

Why

In DeepSeek Harness dark mode, the plugin's selection toolbar and other Side Chat surfaces could render with their light fallback backgrounds while inheriting the host's light foreground color. This produced white or near-white text on a white surface and made the controls unreadable.

The current Harness theme publishes colors through semantic aliases such as --dsw-alias-bg-layer-2, --dsw-alias-label-primary, and --dsw-alias-border-l2. Using those aliases keeps the plugin synchronized with both light and dark palettes without hard-coding a separate dark theme.

User impact

The Side Chat panel, selected-text action toolbar, annotation editor, message bubbles, and interactive states now remain readable in both light and dark appearance modes.

Validation

• npm run typecheck
• npm run lint
• npm run test (76 tests)
• npm_config_cache=/private/tmp/dsh-side-chat-npm-cache npm run pack:verify
• real-browser visual smoke test with the Harness rc.6 theme sheets:
  • dark: rgb(44, 44, 46) surface / rgb(249, 250, 251) text (13.34:1 contrast)
  • light: rgb(255, 255, 255) surface / rgb(15, 17, 21) text

[AHGGG]

Thanks for your contribution! it looks like the issue still persists

[AHGGG]

Maybe we need to add @deepseek-ai/dsh-client-ui-theme to devDependencies, I'll fix it

[AHGGG]

Could you please verify that dark mode works on your machine? I'll merge this PR if everything looks good.