Inline VS Code auth popup opens about:blank on first attempt

[arzafran]

Tracking upstream cmux manaflow-ai#6649 — applicable to our fork but needs a product/scope decision.

Assessment: The code path (BrowserPopupWindowController, BrowserUIDelegate.createWebViewWith at BrowserPanel.swift:6352) exists. The plausible fix (defer makeKeyAndOrderFront until first meaningful navigation commits) is identified but has not been confirmed as the root cause — no upstream fix exists yet, and reproducing requires the full VS Code serve-web flow. Verification against the actual VS Code OAuth window.open pattern is needed before committing to the fix approach.

Upstream: manaflow-ai#6649

[arzafran]

Decision: Investigate, then fix (paired with #21). Root cause not yet confirmed.

Candidate: defer makeKeyAndOrderFront in BrowserUIDelegate.createWebViewWith (BrowserPanel.swift:6352) until the first meaningful navigation commits. Reproduce against the real VS Code serve-web OAuth window.open flow (unblocked once #21 gives a stable serve-web) before committing. Cleared needs-decision.

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