Agent manager now with mouse support

Mouse support shipped in v0.38.0. The wheel moves the cursor, a click selects a row, and a double click focuses a session or folds a group. Click the list, or the mouse back button, to come back. The divider drags, and a click on the messages card opens it. The interaction model started with fruch.

The note skips one detail. Enter focuses by default, and Settings can swap it with attach. A double click still focuses a session either way.