Dock: drop dead pallet-width setters, split on_dock, share span maths

- Only docked pallets have a width divider, so the Width drag and
  WidthReset no longer carry a branch for an undocked pallet; remove the
  then unused DockState::set_width / reset_width. Per-pallet widths are
  still read (older configs, floating size). Tests now size groups.
- on_dock hands hover messages to dock_hover and the release to
  dock_drag_release.
- dock_slot_spans and dock_landing share dock_open_spans.

No behaviour change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013RyGiMXcsf9HTKfqDxaQGt

src/app/dock/tests.rs

@@ -186,7 +186,7 @@ fn dock_icon_press_switches_the_shown_group() {
     assert_eq!(app.dock_shown_group(DockSide::Left), Some(1));
     assert_eq!(app.dock_slot_spans(DockSide::Left), vec![(0, 0.0, 900.0)]);
     // Each group keeps its own width.
-    app.dock.set_width(PanelId::Browser, 400.0);
+    app.dock.set_group_width(DockSide::Left, 1, 400.0);
     assert_eq!(app.dock_column_width(DockSide::Left), 400.0);
     let _ = app.on_dock(DockMsg::IconPress(PanelId::BlockPalette));
     let _ = app.on_dock(DockMsg::DragRelease);