fix(toolbar): persist customizations across window opens (#1455)

[datlechin]

Summary

• Closes Toolbar customizations are not persisted #1455 — toolbar customizations (drag/drop items in Customize Toolbar...) were lost on every session window reopen.
• Two compounding issues in MainWindowToolbar.swift: the toolbar identifier embedded a fresh UUID per instance (so AppKit's autosave key was never the same twice), and autosavesConfiguration was explicitly set to false.
• Use a single stable identifier com.TablePro.main.toolbar shared by every session window, and set autosavesConfiguration = true. AppKit handles read/write to UserDefaults under NSToolbar Configuration <identifier> automatically. This matches the documented pattern in Mail/Safari/Finder/Xcode.
• The comment claiming the UUID prevented tab-group merging was incorrect: tab grouping is driven by NSWindow.tabbingIdentifier (already set per-connection at TabWindowController.swift:60), not by the toolbar identifier. The per-instance hostingControllers retention on each MainWindowToolbar continues to prevent the NSHostingController orphan-view problem.

Test plan

• Open a session window, right-click the toolbar → Customize Toolbar..., drag the "Export & Import" group in, click Done.
• Close the window, reopen it — the group should still be there.
• Open two windows for the same connection (so they tab together), customize one, switch tabs, close one. Confirm the remaining tab's toolbar still renders every item.
• Quit and relaunch — the customized layout should still be there.
• Use the customize sheet's Default Set button — the layout should reset to toolbarDefaultItemIdentifiers.
• xcodebuild test -only-testing:TableProTests/MainWindowToolbarValidationTests passes (three new tests added covering identifier stability, autosave configuration, and the allowed-items-superset invariant).