export: expressive destination buttons, and a receipt that says where it went

The two destinations were a filled Button over an OutlinedButton, and the
success line said "Exported 1 list" and nothing else.

Now they are an M3 Expressive ButtonGroup — equal-weight tiles that grow and
squeeze their neighbour on press, corners morphing square — and the outcome
carries the SAF Uri it was aimed at, so the receipt is a tinted row naming
the folder or zip, with an Open button when something on the device answers
for it. Failures get the same treatment.

app/src/main/res/values/strings.xml

@@ -274,6 +274,7 @@
     <string name="export_to_folder">Save to a folder</string>
     <string name="export_to_zip">Save as a zip file</string>
     <string name="export_running">Exporting…</string>
+    <string name="export_open">Open</string>
     <plurals name="export_done">
         <item quantity="one">Exported %1$d list</item>
         <item quantity="other">Exported %1$d lists</item>