fix(opencode): normalize embedded WebUI paths on Windows

[stenci]

Issue for this PR

Closes #19332

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Windows desktop dev was failing after the embedded WebUI change because packages/opencode/script/build.ts generated opencode-web-ui.gen.ts with backslashes in string literals.

That broke generated code like assets\xml-... / C:\...\assets\xml-..., where \x is treated as the start of an escape sequence by Bun while parsing the generated module.

This changes the generated import paths and file map keys to use forward slashes, which keeps the generated module valid on Windows while preserving the same file mapping.

How did you verify your code works?

I reproduced the failure on Windows by updating local dev to current upstream/dev and running start_desktop.cmd.

Before this change, desktop startup failed with a syntax error in generated opencode-web-ui.gen.ts.

After this change, bun run dev:desktop gets past that generated-file syntax error and desktop dev starts correctly.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[stenci]

The typecheck failure looks unrelated to this PR.

The only file changed here is packages/opencode/script/build.ts, but the failing check is @opencode-ai/desktop:typecheck on packages/desktop/src/i18n/index.ts importing packages/app/src/i18n/pl.

Recent dev typecheck runs are green, so this seems like a flaky/CI-only issue in generated .ts-dist output rather than something caused by this Windows path fix.

Could someone rerun the typecheck job?