Dock: review fixes - hover, preview width, stale indexes, config healing

Fixes:
- Hover state no longer drops when the pointer moves up from one strip
  icon, group grip or title bar to the one above: iced handles the
  element above first, so the old element's "left" (None) cleared the
  hover just set. Leave messages now carry the element and only clear
  their own hover.
- The drop preview used the pallets' own widths; it now uses the
  group's width, matching where the pallet lands.
- Group indexes from messages (group grab, splitter) are looked up
  safely, so a message from before a layout change no longer panics.
- Floating placements read from the config are healed (finite, positive,
  at least the minimum size), like the group weights already were.
- The shown group's edge bar is centred in its row: a stack takes its
  height from its first layer, and the icon column was shorter than the
  row. The "Drag to move group" tooltip is removed.

Simplifications (no behaviour change):
- dock_open_at_default / dock_unpeek replace seven copies of "dock on
  the right if unplaced, then reveal" and four of "stop peeking".
- The drop target takes the grab offset as a parameter, so a drag
  update stores its state once.
- dock_set_open, ensure_settings, the shown-index bookkeeping after a
  group is removed and the on-screen width clamp each lose duplication.
- VBarLabel drops options its single caller never varied.
- The drop preview's landing computation is one helper.

Naming and consistency: pallet names in the + menu, the drag ghost and
the floating title bar go through t!(); StripHit::Tab becomes
StripHit::Join; stale comments updated.

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

src/app/update/mod.rs

@@ -2580,12 +2580,7 @@ impl OpenCADStudio {
             Message::ToggleXrefManager => {
                 self.show_external_references ^= true;
                 if self.show_external_references {
-                    use crate::app::config::DockSide;
-                    use crate::ui::dock::PanelId;
-                    if !self.dock.is_placed(PanelId::ExternalReferences) {
-                        self.dock.dock(PanelId::ExternalReferences, DockSide::Right, usize::MAX);
-                    }
-                    self.dock_reveal(PanelId::ExternalReferences);
+                    self.dock_open_at_default(crate::ui::dock::PanelId::ExternalReferences);
                     self.refresh_xref_manager();
                 }
                 Task::none()