some more refactoring

- UIState.h: Replaced C-style arrays bool[] and ImVec4[] with
std::array<bool, kMaxChannels> and std::array<ImVec4, kMaxChannels> for
type safety and STL compatibility.
  - Application.cpp: Extracted duplicated DPI calculation (lines 189-196
and 475-481) into a systemDpiScale() helper method, reducing both call
sites to single-line calls.
  - Application.cpp: Replaced magic 1400, 900 window dimensions with
kDefaultWindowWidth/kDefaultWindowHeight constants (defined in Types.h).
  - AudioEngine.cpp: Named the magic epsilon thresholds 1e-20f →
kLinearEpsilon and 1e-30f → kLogGuard with explanatory comments, defined
in an anonymous namespace.
  - Types.h: Added kDefaultWindowWidth and kDefaultWindowHeight
constants.

src/dsp/WindowFunctions.cpp

@@ -4,8 +4,6 @@
 
 namespace baudmine {
 
-static constexpr double kPi = 3.14159265358979323846;
-
 void WindowFunctions::generate(WindowType type, int size, std::vector<float>& out,
                                float kaiserBeta) {
     out.resize(size);