feat(camera): comprehensive camera system (C1)

Rebuild the camera around a single persistent MainCamera and the
Camera System design-doc model: two framing modes (Orbit = inspection/
docked tactical, Follow = track the ship) plus Cinematic as a boolean
overlay (free rotation + HUD hide + live gameplay), not a third mode.

Critical fixes (gap analysis A1-A3):
- A1: scenes now *reconfigure* the one startup camera instead of spawning
  a second one; despawn_in_system_scene no longer destroys MainCamera, so
  the session never loses its camera. .single_mut() on MainCamera now
  succeeds during flight.
- A2: Cinematic is a real overlay — toggle (KeyC), free-look rotation in
  any framing, HUD hidden while active, gameplay keeps running.
- A3: removed dead FollowCamera component + setup_follow_camera; tracking
  is unified in track_camera_target.

Gaps B1-B6:
- B1: adopted doc model (Cinematic overlay, not exclusive mode).
- B2: canonical isometric tactical_rotation() baseline.
- B3: smooth reframing via OrbitFocusGoal exponential-damp tween.
- B4: non-zero CameraState defaults.
- B5: consolidated the three orbit-control impls into one (dropped the
  starting_base local control + its Euler variant).
- B6: track_camera_target keeps OrbitCamera.target synced to the focus
  entity every frame.

Docked view now frames the actual station at a tactical iso distance.
cargo check + clippy clean for all newly-authored code; net -10 lines
(more dead code removed than added). 42 tests pass.

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

@@ -21,7 +21,7 @@ pub use docked::{DockedState, UndockEvent};
 pub use flight::{FlightState, FlightControlsPlugin};
 pub use scene::ActiveSystem;
 pub use target::{Targetable, TargetKind, SelectedTarget, TargetSelectionPlugin};
-pub use actions::{ContextualActionPlugin, ActionType, ActionTriggeredEvent};
+pub use actions::{ContextualActionPlugin, ActionType, ActionTriggeredEvent, ActionUi};
 pub use operations::{TimedOperationPlugin, OperationKind, ActiveOperation};
 
 pub struct InSystemPlugin;