feat: add dpi detection and configuration for mouse distance

Implement automatic DPI detection from EDID display data and a manual `mouse_distance_dpi` config option to convert mouse movement pixels into real-world centimeters. The new `calculate_pixels_per_cm()` function in main.c falls back to manual DPI override, then EDID-based auto-detection with sanity checks, and finally a default 96 DPI (37.8 px/cm). The `Config` struct gains a `mouse_distance_dpi` field parsed from config files, and `DWNState` stores the computed `pixels_per_cm` and a `dpi_auto_detected` flag. The panel renderer now uses `dwn->pixels_per_cm` instead of the hardcoded 37.8 constant for mouse distance display.

include/config.h

@@ -39,6 +39,7 @@ struct Config {
     int panel_height;
     int gap;
     char font_name[128];
+    double mouse_distance_dpi;
     ColorScheme colors;
 
     float default_master_ratio;

include/dwn.h

@@ -215,6 +215,9 @@ typedef struct {
     int last_mouse_x;
     int last_mouse_y;
     bool mouse_tracking_initialized;
+
+    double pixels_per_cm;
+    bool dpi_auto_detected;
 } DWNState;
 
 #define PHASE_OFFSET_PANEL_BG      0.0f