chore: set cssFilename to make ecosystem-ci pass

[sapphi-red]

Description

Vite 6 now aligns the CSS file name with the JS file name for lib mode (vitejs/vite#18488).
I set cssFilename to make the repo compatible with Vite 6 and pass the ecosystem-ci (only partially as vite-plugin-svelte for Vite v6 is not released yet).

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• If it's a new feature, provide a convincing reason to add it. Ideally, you should open a suggestion issue first and have it approved before working on it.
• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

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histoire

pnpm add https://pkg.pr.new/histoire-dev/histoire@768

@histoire/app

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/app@768

@histoire/controls

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/controls@768

@histoire/plugin-nuxt

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/plugin-nuxt@768

@histoire/plugin-percy

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/plugin-percy@768

@histoire/plugin-screenshot

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/plugin-screenshot@768

@histoire/plugin-svelte

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/plugin-svelte@768

@histoire/plugin-vue

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/plugin-vue@768

@histoire/plugin-vue2

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/plugin-vue2@768

@histoire/shared

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/shared@768

@histoire/vendors

pnpm add https://pkg.pr.new/histoire-dev/histoire/@histoire/vendors@768

commit: 5a29a3b