Make sidebar collapsible by drag

[ComputelessComputer]

Use the resizable panel collapse behavior and preserve the last expanded width when snapping closed.

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Note

Low Risk
Desktop main-layout UI only; behavior is covered by new unit tests with no auth, data, or backend changes.

Overview
Users can drag the left sidebar resize handle past the minimum so the panel snaps closed, matching collapsible resizable-panel behavior instead of only using the chrome toggle.

ClassicMainBody wires the left panel as collapsible with collapsedSize 0, an imperative ref, and onCollapse that calls leftsidebar.setExpanded(false) after restoring the last expanded width via CSS variables and panel.resize. A lastExpandedLeftSidebarPanelSizeRef keeps the pre-collapse percentage so layout commits and reopening (Show sidebar) resize back to that width before toggleExpanded, without calling expand() on the handle.

Tests extend the resizable mock with forwardRef, onCollapse, and panel handle stubs, and add cases for drag-to-collapse and reopen resize.

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[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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