fix: reopened sign-in presenter from settings

[jacobcxdev]

Summary

• Opened the main window before presenting the existing sign-in sheet from Settings.
• Preserved the existing authentication flow, sheet ownership and account storage.
• Prepared version 4.1.4 (52).

Evidence

Settings previously changed shared sheet state without opening the only window that owns the sheet. A disposable native fixture using the production views reproduced the missing presentation after closing that window. Reopening the window before requesting the sheet restored visible presentation.

The background fixture retained prohibited activation policy and unchanged foreground process. It verifies the presentation ordering, not every SwiftUI scene lifecycle. Apple's documented openWindow action targets the existing singleton main scene. Independent review found no blockers in this narrow route change.

A separate exact Window/Settings scene fixture compiled, but prohibited activation prevented its initial scene creation (windows=[]). That attempt is inconclusive, not a passing scene-level regression. No foreground activation or real account operation was used to force the test.

Verification

• Debug build and full suite passed with release version 4.1.4 (52): 143 tests / 149 runs, zero failures, skips or runtime warnings.
• Release identity and mutation contracts passed.
• No real sign-in, sign-out, credential access, global keyboard event or installed-app preference change occurred.