fix: web chat polish, diff whitespace, and external editor discovery (#685)

[rynfar]

Summary

Adopts the Group A web UI and interaction polish changes from upstream T3 Code as part of upstream integration cycle #685:

• Confirmation dialogs: Wrap long titles safely in confirmation dialogs (upstream #12571).
• Citation popover dismissal: Retain citation comment draft when popover is dismissed (upstream #10831).
• Pull request diff whitespace: Honor whitespace settings in pull request diff views (upstream #12438).
• External editor discovery: Detect installed editors outside PATH across macOS, Linux, and Windows (upstream #12439), with Cursor classic IDE mode launch support (upstream #11498).
• Responsive chat headers: Keep narrow chat headers readable and controls aligned (upstream #12453).

Note: Deferred upstream commit dfbb11bdd7 (fix(web): show plain text in collapsed thought previews) until upstream feature feat(chat): show provider thinking traces (#11784 / 052c7ae53e) is integrated in a dedicated review group.

Verification

• Tested across 5 test suites (73 passing tests).
• Full monorepo typecheck passed cleanly (pnpm typecheck).
• Adversarial audit passed with 0 brand regressions, clean Effect service conventions, and cross-platform safety.

Part of #685.

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Project	Deployment	Actions	Updated
pylon-marketing	Building	Preview	Sep 20, 2026 7:44pm UTC