fix: sidebar interaction blocked by canvas area

[190km]

Summary

• The canvas_content Area in app.rs was positioned at Pos2::ZERO (screen coordinate origin), causing it to span the entire screen including the sidebar region
• Since this Area is rendered at Order::Middle (above the sidebar panel at the base layer), it intercepted pointer events — such as right-clicks — meant for sidebar items whenever terminal panels on the canvas overlapped the sidebar area
• Changed fixed_pos(Pos2::ZERO) to fixed_pos(canvas_rect.min) so the Area only covers the canvas region, allowing all sidebar interactions to work correctly

Test plan

• Open Void with multiple terminal panels, positioning at least one so it overlaps with or is near the sidebar area
• Right-click on sidebar items (workspace entries, terminal list items) — context menus should appear
• Left-click sidebar items to switch workspaces/focus terminals — should work reliably
• Verify canvas interactions (pan, zoom, drag panels, resize panels) still work correctly
• Verify minimap overlay still renders and functions properly

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