Fix Linux in-app browser window-open routing

[psypeal]

Summary

• Enable target-blank popups for ordinary Linux Browser sidebar webviews.
• Normalize Linux HTTP(S) window.open requests with Electron's other disposition into the upstream foreground-tab route.
• Preserve temporary conversation routing and dedicated tabs for Bing result redirects without forcing Google results into new tabs.
• Remove the obsolete renderer-side non-local navigation override.

Root cause

Stock Linux Electron does not expose the proprietary web-contents adoption path used by the upstream desktop runtime. Target-blank result links can therefore arrive with the other disposition and be denied before the upstream tab message is emitted. A broad window_open fallback also incorrectly makes Google results dedicated tabs.

This change keeps the upstream route and adoption behavior intact, adds the narrow Linux disposition normalization, and uses Bing's /ck/a result redirect as the fallback discriminator when Electron does not provide a reliable opener URL.

Scope

This PR contains only window-open behavior and the obsolete navigation-patch cleanup. Linux Browser sidebar user-agent normalization remains isolated in #1022.

Validation

• node --test scripts/patch-linux-window-ui.test.js — 406/406 passing.
• Live isolated-app validation: Bing results open a distinct tab; Google results navigate the retained current tab.
• Fresh upstream DMG 26.707.91948 candidate accepted with all four window-open patches applied.
• The packaged window-only asset contains no user-agent normalization helper or fallback mutation.

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