don't allow UI scale < 100%

Both Windows and Linux installs fail to render properly when the scale
is below 100%:

https://anki.tenderapp.com/discussions/ankidesktop/38909-user-interface-size

Retina Macs do render properly, but they tend to display at appropriate
size out of the box anyway.

qt/aqt/profiles.py

@@ -485,7 +485,8 @@ please see:
     ######################################################################
 
     def uiScale(self) -> float:
-        return self.meta.get("uiScale", 1.0)
+        scale = self.meta.get("uiScale", 1.0)
+        return max(scale, 1)
 
     def setUiScale(self, scale: float) -> None:
         self.meta["uiScale"] = scale

qt/designer/preferences.ui

@@ -246,7 +246,7 @@
             <string>%</string>
            </property>
            <property name="minimum">
-            <number>50</number>
+            <number>100</number>
            </property>
            <property name="maximum">
             <number>200</number>