Decide which panel should be active for a dock based on ordering panels (#22346)

This means that `workspace::ToggleRightDock` will open the assistant if
no right-dock panel has been manually activated, instead of the chat as
before. Also cleans up the `active_panel_index` logic a bit.

cc @nathansobo 

Release Notes:

- Make `workspace::ToggleRightDock` open the assistant panel if no
right-dock panel has yet been activated

crates/project_panel/src/project_panel.rs

@@ -4252,6 +4252,10 @@ impl Panel for ProjectPanel {
                 .map_or(false, |entry| entry.is_dir())
         })
     }
+
+    fn activation_priority(&self) -> u32 {
+        0
+    }
 }
 
 impl FocusableView for ProjectPanel {