Detached windows discard unsaved query changes without asking

[erikdarlingdata]

#462 gave query tabs an unsaved-changes prompt on tab close and window close. Detached windows dodge both.

Detach to Window on a dirty query tab moves the session into a free-floating window via DetachedWindowHelper.ShowDetached. Closing that window destroys the content with no prompt — the close path is the helper's own, and it never asks anyone anything. The tab is also out of MainTabControl.Items while detached, so TabsWithUnsavedChanges() can't see it either: the main window's shutdown prompt walks the tab strip and honestly reports nothing to save while your edit sits dirty in another window.

The helper already takes an onClosing callback, and UnsavedChangesDialog.ShowAsync takes any Window as owner, so the pieces exist — nothing is wired.

Scope worth being explicit about:

• Closing the detached window directly must prompt Save / Don't Save / Cancel, with Cancel keeping the window open. Same rules as Warn about unsaved query changes, and mark modified tabs #462: save-in-place when the session has a file, Save As when it doesn't.
• App shutdown closes every detached window from MainWindow.OnClosed — after the main window is already gone, which is a bad moment to raise a dialog. At minimum that path must not hang or crash; whether it can reasonably prompt is a design question for the implementation to answer and defend.
• Plan and Query Store content also detach through this helper and are read-only; they must keep closing silently.

[erikdarlingdata]

Merged to dev in #477.

Closing a detached window holding a modified query now asks Save / Don't Save / Cancel, owned by that window rather than the main one, with the Save As picker taken from the same window. Re-dock never asks — the content is being moved, not destroyed.

The second half mattered more than the first: a detached session is off the tab strip, so the shutdown prompt was walking tabs and honestly reporting nothing to save while a dirty edit sat in another window. Tabs and detached windows are now one ordered walk, asked while every window is still up — which is what lets OnClosed force the strays shut without raising a dialog over a main window that no longer exists.

Plan and Query Store windows detach through the same helper and are read-only; their close path is byte-for-byte unchanged, pinned by a test.

Known gap, stated plainly: which IStorageProvider the Save As picker receives is not covered — a file picker isn't drivable headlessly, so that one line rests on reading the code.