Fix deprecated syntax failure in publish check

[peter-jerry-ye]

Summary

• remove use of drop([window, app, exit_code]) in src/main/main.mbt
• avoid deprecated implicit empty-trait implementation warning during moon check --deny-warn
• keep behavior by explicitly ignoring the return value of QApplication::exec()

Why

publish-package failed in upstream run due deprecated_syntax errors from implicit Drop implementation when calling drop(...).

Validation

• Local: moon check --target native --deny-warn --package-path src/qpainter
• Fork CI publish run: moon check (native) now passes
  • https://github.com/peter-jerry-ye/qpainter.mbt/actions/runs/23035534074
• In that fork run, only final moon publish fails because token is empty on fork (SECRET not set), expected.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve Qt object lifetimes during event loop

This change removes the only post-exec() use of app and window, so those Ref objects are now eligible to become unreachable before QApplication::exec() returns; in this codebase, Ref<T> uses a finalizer that runs ~T() when collected, so a GC/RC cycle during the event loop can destroy the live QApplication/Window and cause crashes or lost callbacks while the UI is running. Keeping explicit references alive until after exec() returns is required for correctness.

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