fix(canvas): Save button no-op due to leaked click event

The Toolbar and Sidebar Save buttons wired onClick={onSave} directly, so
React passed the MouseEvent as the first argument. handleSave treats its
first arg as a designIdOverride, corrupting design_id and making the save
fail silently. Ctrl+S worked because it calls handleSave() with no args.

Wrap both handlers as onClick={() => onSave()} so no event leaks through.

Add regression tests asserting onSave is called with zero arguments.

Fixes #186

ha-relevant: no

frontend/src/components/panels/Sidebar.tsx

@@ -257,7 +257,7 @@ export function Sidebar({ onAddNode, onAddGroupRect, onAddText, onScan, onZigbee
           icon={Save}
           label="Save Canvas"
           collapsed={collapsed}
-          onClick={onSave}
+          onClick={() => onSave()}
           badge={hasUnsavedChanges}
           accent
         />