cmake: add WITH_CGAL / WITH_VIEWER modes, remove unused Boost

Restructures the CMake build into three clearly separated modes:

  (default)        tests only – Eigen + GTest, no heavy deps
  -DWITH_VIEWER=ON builds viewer lib (libigl / GLFW / GLAD)
  -DWITH_CGAL=ON   builds conformallab_core CLI app;
                   implies WITH_VIEWER automatically

Changes:
- code/CMakeLists.txt: viewer and core targets are now guarded by
  option() flags; GTest / tests target is always built
- code/deps/Cmakelists.txt: CGAL, libigl, glfw, glad are only
  extracted when the corresponding mode is enabled; Eigen always
- Remove Boost (211 MB tarball, 0 usages in the codebase – CGAL 6.x
  does not require Boost for Simple_cartesian / Surface_mesh / basic IO)
- Remove code/src/main.cpp (dead file, not referenced in CMake)

All 16 C++ tests pass in tests-only mode (verified locally).

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

code/src/main.cpp

@@ -1,40 +0,0 @@
-#include <iostream>
-#include <CGAL/Simple_cartesian.h>
- 
-typedef CGAL::Simple_cartesian<double> Kernel;
-typedef Kernel::Point_2 Point_2;
-typedef Kernel::Segment_2 Segment_2;
- 
-int main()
-{
-  Point_2 p(1,1), q(10,10);
- 
-  std::cout << "p = " << p << std::endl;
-  std::cout << "q = " << q.x() << " " << q.y() << std::endl;
- 
-  std::cout << "sqdist(p,q) = "
-            << CGAL::squared_distance(p,q) << std::endl;
- 
-  Segment_2 s(p,q);
-  Point_2 m(5, 9);
- 
-  std::cout << "m = " << m << std::endl;
-  std::cout << "sqdist(Segment_2(p,q), m) = "
-            << CGAL::squared_distance(s,m) << std::endl;
- 
-  std::cout << "p, q, and m ";
-  switch (CGAL::orientation(p,q,m)){
-  case CGAL::COLLINEAR:
-    std::cout << "are collinear\n";
-    break;
-  case CGAL::LEFT_TURN:
-    std::cout << "make a left turn\n";
-    break;
-  case CGAL::RIGHT_TURN:
-    std::cout << "make a right turn\n";
-    break;
-  }
- 
-  std::cout << " midpoint(p,q) = " << CGAL::midpoint(p,q) << std::endl;
-  return 0;
-}