Extract TFlatDropDown reusable dropdown component

Moves CTCSS tone picker and FM step picker from inline TPanel+TFlatButton
arrays into a standalone FlatDropDown.pas module. TFlatDropDown owns the
popup panel, handles positioning and item selection, and fires OnSelect.
ApplyStyle syncs button colors on theme change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ewsdr.lpi

@@ -115,6 +115,10 @@
         <Filename Value="FlatButton.pas"/>
         <IsPartOfProject Value="True"/>
       </Unit>
+      <Unit>
+        <Filename Value="FlatDropDown.pas"/>
+        <IsPartOfProject Value="True"/>
+      </Unit>
       <Unit>
         <Filename Value="AppTheme.pas"/>
         <IsPartOfProject Value="True"/>