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.

CHANGELOG.md

@@ -31,6 +31,20 @@ upload needs a **higher `versionCode`** than every previous upload.
 
 ### Added
 
+- **A two-pane layout for large screens.** A window at or above Material's "expanded"
+  breakpoint — 840 dp, which a 10-inch tablet in landscape at 1280 dp clears — now splits
+  into a master pane and a detail pane. Tapping a row highlights it, and the second pane
+  answers "what is in there?" without a navigation step: a folder's children, or a preview
+  plus the metadata for a file. Opening therefore moves to a double tap **on wide windows
+  only**, because once there is somewhere to show a selection a single tap has to mean
+  "show me"; phone (~411 dp) and 7-inch-portrait (~600 dp) windows keep the layout and the
+  tap behaviour they already had. The root screen previews the main volume on arrival
+  rather than leaving the second pane empty.
+
+  Found while taking the tablet screenshots, where it turned out the app had **no**
+  large-screen layout at all: on a 2560×1600 canvas every row was 2560 px wide with its
+  content stranded in the left ~240 px.
+
 - **M10 — store assets.** The app is named **Plain Files** (one string; the `applicationId`
   was already locked in M0). `tools/make-store-assets.sh` renders the 512×512 RGBA icon and
   the 1024×500 RGB feature graphic from the app's own icon vectors, so the store icon *is*
@@ -42,9 +56,13 @@ upload needs a **higher `versionCode`** than every previous upload.
   declaration requires. The feature graphic's copy was verified to survive Play's
   centre-square crop by cropping the rendered PNG, not by trusting the arithmetic.
 
-  **Not finished:** the screenshots themselves. The generator and the raw-capture
-  directories are in place, but no captures have been taken, so `store-assets/screenshots/`
-  is still empty. The listing is publishable except for that.
+  **Screenshots:** 5 phone shots at 1080×2160, 4 on a 7-inch tablet at 1200×1920 and 4 on a
+  10-inch tablet at 2560×1600 — all 24-bit RGB with no alpha, inside Play's 2:1 cap. The
+  tablet sets come from AOSP AVDs, because there was no tablet hardware to hand. The
+  generator now measures every capture and refuses to normalise a blank one: a headless AVD
+  that has not composed the app's first frame still draws its system bars, so a
+  pure-background capture is not obviously empty — and one had already been committed
+  before the check existed.
 
 - **M9 — monetisation.** Decided: a **one-time paid app at €4.49, with a 14-day launch price
   of €2.29**, both set in the Play Console. No ads, no subscription and no in-app purchases —