fix browser icon clipping in settings dropdowns

[mortenn]

Summary

• fix the browser dropdown template in settings so the selected icon is rendered explicitly instead of being squeezed through a nested content presenter
• give the browser selection combo boxes a shared style with enough vertical room to avoid clipping in the closed state
• keep the change focused to the configuration browser dropdowns used for fallback browser and defaults

Test plan

• Build src/BrowserPicker.App/BrowserPicker.App.csproj with -p:Version=1.0.0
• Verify the selected browser icon is no longer cropped in the settings dropdown on the reporter machine
• Spot-check the defaults browser dropdowns in the settings grid and add row

Fixes #222.