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>

viewer/widgets/PowderCalibrationWidget.cpp

@@ -5,7 +5,7 @@
 #include <QPushButton>
 #include <QGridLayout>
 #include <QLabel>
-#include <numbers>
+#include "../../common/JFJochMath.h"
 
 #include "../image_analysis/geom_refinement/AssignSpotsToRings.h"
 
@@ -26,7 +26,7 @@ PowderCalibrationWidget::PowderCalibrationWidget(QWidget *parent) : QWidget(pare
         std::vector<float> rings = CalculateXtalRings(GetCalibrant(), 10);
         QVector<float> q_rings;
         for (float ring : rings) {
-            q_rings.append(2 * std::numbers::pi / ring);
+            q_rings.append(2 * PI / ring);
         }
         emit ringsFromCalibration(q_rings);
     });