fix: inspector border overlap and laggy toggle

[datlechin]

Summary

• bug: Inspector border overlaps the header #228: Remove .toolbarBackground(.hidden) for default connection colors so the system toolbar background covers the NSSplitViewController divider, preventing it from bleeding into the toolbar area.
• bug: Inspector opening is very laggy #229: Defer isPresented.didSet side effects (UserDefaults write + NotificationCenter post) off the animation frame via DispatchQueue.main.async, eliminating the lag on toggle.

Test plan

• Connect to a database with default (no) connection color — verify inspector divider does not extend into the toolbar
• Connect with a colored connection — verify toolbar tint still works
• Toggle inspector (Cmd+I) — verify smooth native animation matching the left sidebar
• Select a row with auto-show inspector enabled — verify inspector opens smoothly
• Open 2+ native tabs — verify inspector state syncs across tabs

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