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/ui/Application.h

@@ -66,6 +66,7 @@ private:
     float     lastDpr_       = 0.0f;
     void      applyUIScale(float scale);
     void      requestUIScale(float scale);
+    float     systemDpiScale() const;
     void      syncCanvasSize();
 
     // UI visibility