StudioMode switch for full-editor capabilities (#23)

This PR adds a `StudioMode` resource to toggle editing capabilities, and
configures the frontend application to read the desired mode from a URL
parameter.

For now, setting the app to `StudioMode::Viewer` with
`<app-url>?mode=viewer` hides the Transform Toolbar and disables most of
the interactive features of the app, but otherwise exhibits the same
behavior as `StudioMode::Editor` / `<app-url>?mode=studio`.

In the future, the "Viewer" and "Editor" modes will receive different
initialization inputs to better serve the needs of their use cases. In
particular:

* The "Viewer" mode will accept a BVH animation handle, which should be
  played in a loop on a simple mannequin asset, allowing a user to preview
  a given animation before using it within the full studio workflow.
* The "Editor" mode will accept a full Bevy scene handle which can be
  edited and saved to the user's account. These scenes will serve as the
  basis for the generative-AI workflows Storyteller Studio is ultimately
  seeking to facilitate.
* It will likely be useful for "Editor" mode to alternatively accept
  only a BVH animation handle, which would be used to initialize a new
  scene for editing similar to "Viewer" mode, but with editing
  capabilities enabled.

studio/src/debug.rs

@@ -7,7 +7,7 @@ use bevy_mod_picking::debug::DebugPickingMode;
 use crate::{
     camera_controller::{CameraController, CameraMotionSystem},
     input::{Action, InputAction, Lifecycle},
-    MainCamera,
+    is_editor, MainCamera,
 };
 
 #[derive(Resource, Deref, DerefMut)]
@@ -36,7 +36,9 @@ impl Plugin for DebugPlugin {
                 Update,
                 (
                     toggle_debug_mode,
-                    update_debug_picking_mode.run_if(resource_changed::<DebugMode>()),
+                    update_debug_picking_mode
+                        .run_if(is_editor)
+                        .run_if(resource_changed::<DebugMode>()),
                     update_gizmo_line_thickness,
                     (draw_world_axes, draw_debug_points)
                         .after(CameraMotionSystem)