Warn about unsaved query changes, and mark modified tabs

[erikdarlingdata]

Reported by @joshdbe in #458 after testing the new Open/Save Query.

A query session has no idea whether it has been edited. You can open a .sql, change it, close the tab or the app, and lose the edit without being asked. Nothing anywhere warns you.

Two symptoms, one missing piece:

1. No prompt on close. Closing a modified tab, or the window, discards the edit silently. Wanted: Save / Don't Save / Cancel, and Cancel actually cancels the close.
2. No modified marker. Josh asked for the VS Code treatment — the tab's × becomes a filled dot while there are unsaved changes, and goes back to × on hover so you can still close it.

What's there now

QuerySessionControl gained SourceFilePath in #459 but tracks nothing about content. The dirty flag is the whole feature; both symptoms are consumers of it.

• Compare QueryEditor.Text against the text as of the last load or save, rather than a bare "was edited" bool — retyping something back to how it started should not leave the tab marked.
• A never-saved scratch tab with content typed into it is also dirty, and its prompt has to go through Save As, not save-in-place.
• The tab header is StackPanel { headerText, closeBtn } in MainWindow.Tabs.cs, so the marker is a swap of closeBtn content plus a pointer-over trigger.
• Window.Closing needs to walk every tab, not just the active one, and cancel the close if anyone picks Cancel.

Plan tabs are read-only and are not part of this.

[erikdarlingdata]

Merged to dev in #469.

Query sessions now track dirty state by comparing the editor against the text as of the last load or save, so undoing back to the original clears the marker rather than leaving a prompt you learn to dismiss. Closing a tab or the window asks Save / Don't Save / Cancel, Cancel actually cancels, and the window walks every tab rather than the selected one — the edit at risk is usually the tab you're not looking at.

The tab's ✕ becomes a filled dot while modified and reverts to ✕ under the pointer, so it stays closable. @joshdbe, that's the VS Code behavior you asked for.

Known gap: a query detached into its own floating window still closes without asking. That's shared plumbing with plan and Query Store content and needs its own issue.