Fix #711: Retain sidebar panel state when switching tabs

[Borisserz]

Description

Fixes navigator/inspector sidebar panels resetting expand and scroll state when switching between sidebar tabs.

WorkspacePanelView previously swapped content with if let selection = selectedTab { selection }, which destroyed and recreated AppKit outline views. It now keeps every tab panel mounted in a ZStack and toggles visibility with opacity / hit-testing, so NSOutlineView state survives.

Applies to both the left navigator and right inspector (they share WorkspacePanelView).

Related Issues

• Closes 🐞 Sidebar tab panel state is not retained when returning to it from another tab #711

Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Screenshots

Please verify locally (UI): expand folders in Project Navigator → switch to Search → switch back → expansion/scroll retained.

Test plan

• WorkspacePanelRetentionTests (all tab bodies stay mounted after selection change)
• SidebarPanelRetentionUITests (expansion survives Project ↔ Search)
• SwiftLint --strict on touched files (0 violations)
• Manual: same flow in Inspector tabs
• CI SwiftLint + tests green

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