feat(app): add query file open & save workflow and top-level tab drag/drop support

[MadeAThing]

• add ability to open (and save) SQL files as well as sqlplan
• add Open Query and Save Query As menu actions with Ctrl+S/Ctrl+Shift+S shortcuts
• expose query text/source path on query sessions to support file-backed tabs
• extend main window file/menu state handling and keyboard routing
• add top-level tab drag/drop plumbing and visual drop cues
• include app settings updates required by new query file behaviors

What does this PR do?

A clear description of the change and why it's being made.

Which component(s) does this affect?

• Desktop App (PlanViewer.App)
• Core Library (PlanViewer.Core)
• CLI Tool (PlanViewer.Cli)
• SSMS Extension (PlanViewer.Ssms)
• Tests
• Documentation

How was this tested?

Describe the testing you've done. Include:

• Plan files tested: estimated, actual, Query Store (but none of this functionality was touched by the changes)
• Platforms tested: macOS
• Open .sql files as well as .sqlplan
• Save .sql files
• Save As .sql files

Checklist

• I have read the contributing guide
• My code builds with zero warnings (dotnet build -c Debug)
• All tests pass (dotnet test)
• I have not introduced any hardcoded credentials or server names

[MadeAThing]

Sorry, Erik! I raised a PR to your repo, not my fork!