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.

docs/PLAY_RELEASE_CHECKLIST.md

@@ -115,8 +115,16 @@ POST_NOTIFICATIONS
 
 Full copy and the generated graphics land in **M10** (`docs/STORE_LISTING.md`). Console
 fields needed: app name, short description, full description, app icon 512×512, feature
-graphic 1024×500, and at least 4 phone screenshots (Play asks for 4+ per large-screen form
-factor too).
+graphic 1024×500, and the screenshots.
+
+- [ ] Upload the screenshots from `store-assets/screenshots/`: **phone** 1080×2160 (5),
+      **7-inch tablet** 1200×1920 (4), **10-inch tablet** 2560×1600 (4). Play wants 4+ per
+      large-screen form factor. All three sets clear its rules — 24-bit PNG with no alpha,
+      shorter side ≥ 320, longer side ≤ 3840, and longer side ≤ 2× the shorter side.
+      `tools/make-store-screenshots.sh` enforces the aspect ratio and rejects a capture
+      whose app area is blank.
+- [ ] Expect the 7-inch set to be single-pane and the 10-inch set two-pane. That is the
+      adaptive layout working, not an inconsistency — see the CHANGELOG.
 
 ## 7. Release tracks and pricing