portability: replace M_PI / std::numbers::pi with a host+device-safe PI

M_PI is a POSIX <math.h> extension that MSVC does not define without
_USE_MATH_DEFINES. std::numbers::pi (introduced in the viewer guard commit) is
C++20, but CUDA here is compiled as C++17 (CMAKE_CUDA_STANDARD 17) and several
common/ headers are pulled into .cu device translation units, so std::numbers is
not available there.

Add common/JFJochMath.h with a dependency-free `constexpr double PI` that works
in host code (including MSVC), in CUDA device code, and under C++17/20, and use
it everywhere:
- common/ and image_analysis/ (incl. CUDA .cu): 78 M_PI occurrences, 22 files
- broker/OpenAPIConvert.cpp
- viewer/: the 5 files that used std::numbers::pi now use PI, for one consistent
  convention across the codebase

Verified to build: JFJochImageAnalysis (incl. CUDA), jfjoch_viewer, JFJochBroker.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common/DiffractionGeometry.cpp

@@ -1,6 +1,7 @@
 // SPDX-FileCopyrightText: 2025 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
 // SPDX-License-Identifier: GPL-3.0-only
 
+#include "JFJochMath.h"
 #include <cmath>
 
 #include "DiffractionGeometry.h"
@@ -48,7 +49,7 @@ float DiffractionGeometry::Phi_rad(float x, float y) const {
     auto lab = LabCoord(x, y);
     auto v = atan2f(lab.y, lab.x);
     if (v < 0)
-        v += 2.0f * M_PI;
+        v += 2.0f * PI;
     return v;
 }
 
@@ -58,7 +59,7 @@ float DiffractionGeometry::PxlToRes(float x, float y) const {
 }
 
 float DiffractionGeometry::PxlToQ(float x, float y) const {
-    return 2.0f * M_PI / PxlToRes(x,y);
+    return 2.0f * PI / PxlToRes(x,y);
 }
 
 float DiffractionGeometry::PxlToRes(float dist_pxl) const {
@@ -87,7 +88,7 @@ float DiffractionGeometry::DistFromEwaldSphere(const Coord &recip) const {
 }
 
 float DiffractionGeometry::CalcAzIntSolidAngleCorr(float q) const {
-    float sin_theta = q * wavelength_A / (4 * static_cast<float>(M_PI));
+    float sin_theta = q * wavelength_A / (4 * static_cast<float>(PI));
     float cos_2theta = 1.0f - 2.0f * sin_theta * sin_theta; // cos(2*alpha) = 1 - 2 * sin(alpha)^2
     float cos_2theta_3 = cos_2theta * cos_2theta * cos_2theta;
     return cos_2theta_3;