Add a two-pane layout for large screens, and re-shoot the tablet sets

A window at or above Material expanded breakpoint (840dp) now splits into a master pane and a detail pane: tapping a row highlights it and the second pane shows a folder children, or a preview plus the metadata for a file. Opening moves to a double tap on wide windows only; phones (~411dp) and the 7-inch in portrait (~600dp) keep their layout and their tap behaviour.

Found while taking the tablet screenshots: the app had no large-screen layout at all, so on a 2560x1600 canvas every row was 2560px wide with its content stranded in the left ~240px.

The tablet sets are re-shot on this build. That also caught a blank capture (7-inch 01-roots) that had already been committed: a headless AVD draws its system bars before Compose composes its first frame, so the file was not obviously empty. make-store-screenshots.sh now measures each capture app area and refuses to normalise one that is blank.

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

@@ -89,6 +89,10 @@
     <string name="browser_details_modified">Modified</string>
     <string name="browser_details_folder">Folder</string>
 
+    <!-- ===== Browser: wide-screen detail pane ===== -->
+    <string name="browser_detail_placeholder">Select an item to preview it</string>
+    <string name="browser_detail_loading">Loading…</string>
+
     <!-- ===== File categories (the "Type" row of the details sheet) ===== -->
     <string name="browser_kind_image">Image</string>
     <string name="browser_kind_video">Video</string>