Polish navigation for native sidebar and detail push

[davidhoang]

Summary

• Remove programmatic sidebar auto-collapse so NavigationSplitView handles expand/collapse natively
• Add a left sidebar toolbar toggle (sidebar.left) matching the inspector control
• Push image detail via NavigationStack + navigationDestination instead of a ZStack opacity overlay
• Persist inspector visibility with @AppStorage and remove custom inspector chrome
• Delete unused WidthReader and update navigation rules in design.md

Test plan

• Resize the window and confirm the left sidebar collapses/expands smoothly via the divider and ⌃⌘S
• Toggle left sidebar and right inspector from the toolbar
• Double-click an image and confirm detail pushes with native back navigation
• Press Escape or click back to return to the library grid
• Confirm inspector visibility persists across detail enter/exit and app relaunch
• Verify VoiceOver labels on sidebar.toggle and inspector.toggle

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