Slots page usability and drawer fixes

Noticed while checking Task 8111 on the live stack (Automation › Slots, dev fixture): 1. Every project bucket and its node sub-bucket open collapsed, so seeing any slot takes two disclosure clicks. Open a project's buckets by default when there are few slots (or remember the last expanded state). 2. At 448px the pinned "Delete slot" footer covers the second line of the drawer's Actions buttons until you scroll. Leave room under the actions (bottom padding equal to the footer's height) so no action is hidden behind the footer. 3. The Actions row's "Provider" select sits taller than the buttons around it (its outlined label pokes above the row). Match its height to the buttons, or move it out of the button row. Out of scope: the drawer's fact list (Task 8111), any API change. Done means: - Each item shipped or declined with a reason; WorktreePool web tests pass. - Screenshots at 1366px and 448px.

Merged #6340 (d51386a). Slot buckets of ≤10 open by default, and both opened and folded choices are remembered. Provider select is level with the buttons. Footer-height padding declined: the footer is in the layout and covers nothing at scroll end; added small padding instead. Visual check done at 1366 and 448. Filed follow-up #8193. Deploy for d51386a was cancelled by a newer develop deploy.