feat: Add customizable accent color setting

Introduce an accent color preference across the app: add a settings UI input (with restart note), persist it to AppState and config (read/write), and accept it in save_settings. Integrate the value into theming: compute a semi-transparent selection color, apply it to table selection styles, and pass it to qdarktheme as the primary color (including when window transparency is enabled). Also apply the accent to the tracker combo selection styling as a UI polish.

src/main.py

@@ -21,13 +21,17 @@ import time
 import sys
 
 def run_gui(app):
-    qdarktheme.setup_theme("auto")
+    custom_colors = {}
+    if state.accent_color:
+        custom_colors["primary"] = state.accent_color
+    qdarktheme.setup_theme("auto", custom_colors=custom_colors if custom_colors else None)
     widget = MainWindow()
     
     # Check OS for window transparency compatibility and apply
     if state.window_transparency and platform.system() != "Windows":
         widget.setAttribute(Qt.WidgetAttribute.WA_TranslucentBackground)
-        qdarktheme.setup_theme("auto", custom_colors={"background": "#00000000"})
+        transparent_colors = {"background": "#00000000", **custom_colors}
+        qdarktheme.setup_theme("auto", custom_colors=transparent_colors)
 
     set_main_window(widget)
     widget.show()