Allow main window to be resized narrower (MinWidth 750 -> 320)

Useful when placing the window beside other content, e.g. next to a
video player. Caption text already wraps, so narrower widths render fine.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jrkasprzyk

src/windows/MainWindow.xaml

@@ -9,7 +9,7 @@
     Title="LiveCaptions Translator"
     Width="750"
     Height="170"
-    MinWidth="750"
+    MinWidth="320"
     MinHeight="170"
     Background="Transparent"
     ExtendsContentIntoTitleBar="True"