Fix widget CSS output filename

[ruibaby]

What changed

• Configure Rslib to emit the widget stylesheet as index.css.
• Keep the generated CSS filename aligned with the path used by CommentWidgetHeadProcessor.

Why

After the Rslib dependency update, the widget build emitted the stylesheet with a generated name such as 59.css, while CommentWidgetHeadProcessor continued to request index.css. As a result, the widget stylesheet could not be loaded.

Validation

• pnpm --filter ./packages/widget build
• pnpm exec biome check packages/widget/rslib.config.mjs
• ./gradlew build