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>

image_analysis/pixel_refinement/PixelRefine.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 "../../common/JFJochMath.h"
 #include "PixelRefine.h"
 
 #include <algorithm>
@@ -499,7 +500,7 @@ void PixelRefine::Run(const T *image,
                                beam, dist_mm, detector_rot, latt_vec0, latt_vec1, latt_vec2,
                                q_sq, eps_r, eps_t_sq))
                 continue;
-            const double P_t = std::exp(-eps_t_sq / (R1 * R1)) / (M_PI * R1 * R1);
+            const double P_t = std::exp(-eps_t_sq / (R1 * R1)) / (PI * R1 * R1);
             const double R0_eff_sq = R0 * R0 + g.R_bw_sq;
             const double P_rad = std::exp(-eps_r * eps_r / R0_eff_sq);
             const double v = std::max(g.Ibkg, 1.0);
@@ -597,7 +598,7 @@ void PixelRefine::Run(const T *image,
                 // Tangential profile shape (area-normalized) -> the fit template, using the
                 // per-reflection R1_eff (Term 2).
                 const double R1 = g.R1_eff;
-                const double P_t = std::exp(-eps_t_sq / (R1 * R1)) / (M_PI * R1 * R1);
+                const double P_t = std::exp(-eps_t_sq / (R1 * R1)) / (PI * R1 * R1);
                 prof_full += P_t; // whole shoebox, on- or off-detector
 
                 // Only real, unmasked detector pixels carry signal.