feat: add system theme detection for sv-ttk

whisper_local/tray/_theme.py

@@ -0,0 +1,12 @@
+"""System-Theme-Erkennung und sv-ttk-Anwendung."""
+
+import tkinter as tk
+
+
+def apply_system_theme(root: tk.Tk) -> None:
+    """Setzt sv-ttk-Theme passend zum Windows-System-Theme (Light/Dark)."""
+    import darkdetect
+    import sv_ttk
+
+    detected = darkdetect.theme() or "Light"
+    sv_ttk.set_theme(detected.lower())