Allow 360° orbit, add Center View button and XYZ reference axes

Camera (apps/game/src/camera.rs):
- Replace yaw/pitch with a single Quat rotation. Mouse drag now does
  yaw-around-world-Y * rotation * pitch-around-local-X, which is gimbal-free
  and allows true 360° tumbling in any direction.
- Remove pitch clamps (ORBIT_MIN_PITCH / ORBIT_MAX_PITCH).
- Add ResetOrbitCamera resource + apply_orbit_reset system that snaps the
  orbit camera back to default on demand.
- OrbitCamera::reset() helper.

Galaxy scene (apps/game/src/gameplay/galaxy_creation/):
- New axes.rs: spawn_axes() draws three emissive cylinders through the
  origin (X=red, Y=green, Z=blue) as children of the scene root so they
  rebuild with the rest of the galaxy.
- UI adds a 'Center View' button below Regenerate that inserts
  ResetOrbitCamera; handled by reset_view_button_handler.
- Help text now mentions drag/zoom/escape.

apps/game/src/gameplay/galaxy_creation/mod.rs

@@ -5,6 +5,7 @@
 //! click-to-select is handled in [`super::selection`]; the slider/info panels
 //! live in [`super::ui`].
 
+mod axes;
 mod params;
 mod selection;
 mod ui;
@@ -13,6 +14,7 @@ use bevy::prelude::*;
 use rand::rngs::StdRng;
 use rand::{Rng, SeedableRng};
 
+use crate::camera::apply_orbit_reset;
 use crate::state::AppState;
 
 pub use params::{GalaxyParams, SelectedStar};
@@ -32,10 +34,12 @@ impl Plugin for GalaxyCreationPlugin {
                     escape_to_main_menu,
                     ui::param_button_handler,
                     ui::refresh_control_panel_values,
+                    ui::reset_view_button_handler,
                     regenerate_galaxy_on_param_change,
                     selection::select_star_on_click,
                     selection::animate_selected_star,
                     selection::refresh_info_panel,
+                    apply_orbit_reset,
                 )
                     .chain()
                     .run_if(in_state(AppState::GalaxyCreation)),
@@ -239,6 +243,9 @@ fn spawn_galaxy_scene(
             GalaxyCreationSpawned,
         ))
         .with_children(|parent| {
+            // XYZ reference axes through the origin.
+            axes::spawn_axes(parent, meshes, materials);
+
             // Star systems.
             for sys in systems {
                 let (mesh, material) = if sys.faction_index == 0 && sys.position.length() < 40.0