🎨 Palette: Replace window.confirm with state-based soft confirmation for End Session

[Sczitzo]

💡 What:
Replaced the thread-blocking window.confirm() native browser dialog in the "End Session" workflow with an inline, state-based soft confirmation UI.

🎯 Why:
Native browser alerts block the main thread, visually disrupt the user context, and are generally jarring UX. A state-based confirmation keeps the user within the flow of the application and provides a more modern, integrated feel while still preventing accidental session terminations.

📸 Before/After:
Before: Clicking "End Session" triggered a system-level popup.
After: Clicking "End Session" transitions the button area into a red confirmation zone with explicit "Cancel" and "Confirm End" buttons, retaining the warning text about generating the session summary.

♿ Accessibility:
Ensured the warning text ("This will generate the session summary.") is rendered inside a paragraph with role="status" so that screen readers will reliably and automatically announce the critical consequence of the destructive action when the confirmation state toggles on. Included focus-visible styling on the new buttons for keyboard navigation.

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PR created automatically by Jules for task 18136600904094008325 started by @Sczitzo

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