Avoid duplicate tray windows

[gaoflow]

Summary

• add a shared helper to reuse an already-open form of the requested type
• bring the existing paste dialog or settings wizard to the front instead of opening duplicates from the tray icon/menu

Fixes #100

Validation

• git diff --check

Not run locally: dotnet restore / Windows build, because this environment does not have dotnet, msbuild, xbuild, or csc installed.

[eltos]

This is an AI generated PR submitted as-is without testing.
There is very limited value in having an external contributor as an intermediary between a maintainer and an AI model. It's also worth emphasizing that all contributions are reviewed by humans with limited available time. AI-driven contributions can easily create a dangerous imbalance where a maintainer invests more time into a PR than the contributor themselves.
Please refrain from opening further PRs against this or any other project.