Unpinning a part in the net pane closes the detail pane while another part is selected #148

In the net pane, a second click on a part row takes its pin off and closes the detail pane. It does this even when a different part is the selected part.

Steps:

1. Click part X on the board. X is selected and the detail pane shows X.
2. Select a net. In the net pane, click part Y. The detail pane now shows Y.
3. Click Y again in the net pane.

Result: Y's pin comes off and the detail pane closes. X is still the selected part: its row in the component list stays marked and the address bar still says focus=comp:X.

Expected: the detail pane and the selected part agree.

Cause: the click handler on net pane rows in populateNetResults (web/app.js) shows a part in the detail pane but never sets selectedFootprintIdx. The unpin path calls renderDetailPane(null, false) whatever part is selected. That handler has never set the selected part, so the mismatch is older than PR #147. That PR added the closing on unpin.

What the pane should show in that case (X again, or nothing with X deselected) is still to be decided.

Found in the review of PR #147.