feat: migrate simulation to physically-based unit system

Replace arbitrary game units with real-world measurements throughout
the simulation and viewer: planet orbits in AU, sizes in km, galaxy
positions in light-years. Add SimulationUnits helpers for conversions,
separate WarpSpeed from FtlSpeed for ships, fix FTL transit progress
to use galaxy-space distances, overhaul Lagrange point placement with
Hill sphere approximation, and update the viewer to scale and format
all distances correctly. Ships in FTL transit now render in galaxy view.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/backend/Contracts/WorldContracts.Ships.cs

@@ -22,6 +22,8 @@ public sealed record ShipSnapshot(
   string? CargoItemId,
   float WorkerPopulation,
   float EnergyStored,
+  float TravelSpeed,
+  string TravelSpeedUnit,
   IReadOnlyList<InventoryEntry> Inventory,
   string FactionId,
   float Health,
@@ -51,6 +53,8 @@ public sealed record ShipDelta(
   string? CargoItemId,
   float WorkerPopulation,
   float EnergyStored,
+  float TravelSpeed,
+  string TravelSpeedUnit,
   IReadOnlyList<InventoryEntry> Inventory,
   string FactionId,
   float Health,