fix: resolve css files in node_modules

[ocavue]

• This PR contains AI-generated code, but I have carefully reviewed it myself. Otherwise, my PR may be closed.

Description

Closes #794

Linked Issues

Additional context

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commit: 3a05faa

[Copilot]

Pull request overview

This PR fixes CSS @import resolution for LightningCSS when importing styles from packages under node_modules (e.g. @import "pkg/path.css"), addressing issue #794.

Changes:

• Add module resolution via oxc-resolver for LightningCSS bundling, enabling node_modules import resolution.
• Extend CSS tests and snapshots to cover @import from a mock package in node_modules for both PostCSS and LightningCSS transformers.
• Add oxc-resolver to the workspace catalog and @tsdown/css package dependencies.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/css.test.ts	Expands @import test coverage to include node_modules-style imports and updates expectations.
tests/snapshots/css/postcss-with-import.snap.md	Updates snapshot output to include inlined .lib rules from the imported package CSS.
tests/snapshots/css/lightningcss-transformer-with-import.snap.md	Updates snapshot output to include inlined .lib rules for LightningCSS bundling.
pnpm-workspace.yaml	Adds oxc-resolver to the pnpm catalog for consistent dependency management.
pnpm-lock.yaml	Locks oxc-resolver (and its platform bindings) and updates dependency graph accordingly.
packages/css/src/lightningcss.ts	Implements oxc-resolver-based resolution inside LightningCSS bundling to support node_modules CSS imports.
packages/css/package.json	Declares oxc-resolver as a dependency of @tsdown/css.

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported

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[ocavue]

@sxzz I've resolved the merge conflict.