Fix meeting prompt dismissal crash

[calebchongc]

Summary

• Close the meeting-prompt window through a native cmd_close_meeting_prompt command that destroys the webview window.
• Make prompt dismissal idempotent so Dismiss, X, Escape, success auto-close, and auto-dismiss all share the same close path.
• Add a regression test covering that the meeting prompt dismissal flow uses the native destroy path.

Fixes #333.

Scope

This branch is intentionally limited to the meeting-prompt crash fix and excludes the call-detection changes from #331.

Tests

• cargo fmt --check
• cargo clippy -p minutes-app --all-targets -- -D warnings
• cargo test -p minutes-app (207 passed)
• ./scripts/install-dev-app.sh --no-open
• Manual: built/installed Minutes Dev.app, exercised meeting-prompt dismissal paths, and confirmed dismissing no longer crashes the app.

Manual test caveat

I could not fully exercise the Join & Record calendar URL path on this managed Mac. The bundled calendar-events helper continued to report write-only Calendar access even though System Settings showed Full Access, so direct helper reads failed with access not granted.

[vercel]

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[silverstein]

Merged. This is the cleaner version you promised when you closed #329, and it shows: routing every dismissal path through one native destroy, deferred out of the JS frame, with the closeScheduled guard on the JS side and a None-check on the native side, closes the WebKit self-close crash without the loose ends. I verified there are no orphaned getCurrentWebviewWindow references left, so no path can hit a missing symbol.

On your manual-test caveat: no worries about the Join & Record calendar path. The code that changed is the close path, which the dismiss/X/Escape/auto-close paths you did exercise all share, so it is well covered. The calendar write-only permission thing you hit is a separate known macOS quirk on this kind of managed Mac, not something your change touches.

That is three merged now (#328, #334) plus the engine direction on #332. Thanks Caleb.