fix: avoid dev app WebKit frame crash

[calebchongc]

Summary

• Keep the long-lived main Tauri WebView opaque and skip macOS vibrancy to reduce WebKit frame-update crashes when the app is hidden/shown from the tray.
• Stop upcoming-meeting calendar polls from mutating hidden main-window UI, and gate Recall auto-expand on the main window being visible and focused before it resizes the WebView.
• Add regression guards for both the main-window opacity/vibrancy settings and the upcoming-meeting auto-expand visibility/focus contract.
• Make the markdown rename collision test timezone-safe after it failed under Asia/Singapore local time.

Testing

• cargo fmt --all -- --check
• cargo clippy --all --no-default-features -- -D warnings
• cargo test -p minutes-core --no-default-features --lib
• cargo test -p minutes-app --no-default-features
• ./scripts/install-dev-app.sh
• Relaunched ~/Applications/Minutes Dev.app, hid the main window, waited 75s for the upcoming-meeting poll interval, and confirmed PID stayed alive with no new minutes-app .ips after the 2026-06-15 14:27:23 pre-fix crash report.

[vercel]

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[silverstein]

Merged. This is your second strong contribution this week, and exactly the kind I want to see: a real crash, a minimal fix, a regression guard that documents the why, and a bonus timezone-safe test fix I didn't even ask for.

One thing worth flagging: making the main window opaque is also the change I'd suspected would fix #312 (Windows 11 menu bar rendering transparent). That one is Windows-specific and I can't verify it from here, so I've asked the reporter to confirm on his machine rather than auto-closing it on a guess. If it's confirmed fixed, your PR gets the credit there too.

Thanks Caleb.