[nikoscham]

Decouple the frontal solver from physics models and restructure src

Frontal solver (#77):

• runFrontalSolver(assembleFront, meshData, boundaryConditionsHandler)
  no longer imports any physics model; model-specific parameters are
  bound into the assembler by the caller
• Boundary condition classes implement a common frontal interface:
  imposeDirichletBoundaryConditionsFront and the optional
  imposeRobinBoundaryConditionsFront (returns null off Robin boundaries)
• newtonRaphson takes assembleFront and boundaryConditionsHandler via
  context instead of importing frontPropagation
• Frontal convection now applies every side of an element on the same
  boundary, matching the matrix assembly

Restructure:

• elements/: basis functions, numerical integration, isoparametric
  mapping, element geometry and initializeFEA (split from meshUtils)
• physics/: models together with their boundary condition classes
  (was models/)
• linalg/: sparse matrix and BLAS utilities (was numerics/)
• io/: Gmsh reader, now including the Gmsh-to-FEAScript conversion
  previously in Mesh.parseMeshFromGmsh
• solvers/index.js exports every solver except WebGPUComputeEngine

Deduplicate boundary conditions:

• Add getBoundarySides() to replace the repeated side-node tables
• Thermal and generic BCs share one node loop for the matrix and
  frontal Dirichlet paths
• Thermal convection uses one per-side computation for both paths
  (thermalBoundaryConditions.js 989 -> 425 lines)

Add Prettier config matching the existing code style