fix(ui_oauth): use dart.library.js_interop for Wasm-compatible imports

[Rexios80]

Description

OAuthProvider selects its platform sign-in implementation with a conditional import gated on dart.library.html. That library is unavailable when compiling to Wasm, so pana/pub.dev resolves the default (platform_oauth_sign_in.dart) which pulls in desktop_webview_auth / dart:io, costing the WASM platform points for firebase_ui_oauth and dependents (e.g. firebase_ui_oauth_oidc).

This PR switches that condition to dart.library.js_interop, which is present for both JS and Wasm web compiles, so the web implementation is selected correctly.

The public library also re-exported desktop_webview_auth unconditionally. Pana's Wasm check follows every public export with no pruning, so that barrel export still pulled dart:io into the graph after the conditional import fix. Desktop types (AuthResult, ProviderArgs, and the Google/Facebook/Twitter sign-in args) are now re-exported only on IO, with web/Wasm stubs that do not import dart:io.

Related Issues

N/A — improves Wasm readiness / pub points for firebase_ui_oauth and packages that depend on it.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• All unit tests pass (melos run test:unit:all doesn't fail).
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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

[gemini-code-assist]

Code Review

This pull request updates the conditional import in oauth_provider.dart to check for dart.library.js_interop instead of dart.library.html. I have no feedback to provide as there are no review comments.