fix: Vite 8 (Rolldown) compatibility for content scripts

[sheldhur]

Summary

Two fixes for Vite 8 (Rolldown) compatibility when building Chrome extensions with content scripts:

1. Duplicate refIds: this.emitFile({ type: "chunk", name: basename(file) }) returns the same refId for different files with the same basename (e.g. seller/content.tsx, cmp/content.tsx, marketplace/content.tsx). Only the first script gets its fileName resolved; the rest fail with Content script fileName is undefined.

2. Wrong MAIN world loader path: The loader uses ./${fileName.split("/").at(-1)} which loses the subdirectory, producing a broken path.

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Latest commit: a19f489

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[sheldhur]

@Toumash can you look at this? I can update the failed tests if you're ready to merge it