Give a video row a frame of the video, and fix what R8 was hiding

A video row showed a film-strip icon, and a phone holds more video than anything else, so a column of identical icons said nothing about which clip was which. Images and videos now take the same path - EntryIcon hands Coil the file, and for a video the request asks for a frame - which cost exactly one dependency: io.coil-kt.coil3:coil-video, at the same version as the Coil already in use, whose own POM adds nothing that Coil did not already bring (coil-core, kotlin-stdlib, core-ktx). No ExoPlayer, no media3, and no new permission: the decoder is the platform's own MediaMetadataRetriever.

Two details are decisions rather than defaults. The frame is taken one second in, because the first frame of a recording is very often black or the middle of a fade-in, and a thumbnail of a black rectangle is no more useful than the icon it replaced. And a decode that fails - an unsupported codec, or a file only pretending to be a video - settles on the icon rather than leaving an empty square, which the row remembers so it is not retried on every recomposition.

The dependency also nearly shipped a feature that did not work. coil-video registers its decoder through a META-INF/services entry, and in a debug APK that is enough: the file and the class it names are both there, and frames decoded. In the release APK, R8 removes both, because a class mentioned only from a resource looks unused to it. So the feature worked in every build that had been looked at and would have failed silently in the one users install - the worst shape a bug can have, since it needs no code change to "fix" and would have been reported as "video thumbnails don't work" against a build nobody had run. The factory is now registered explicitly against an ImageLoader in FileManagerApplication, which makes it a code reference R8 cannot drop, and the release APK was installed and driven to the Videos library to confirm the frames decode there too. That also closes the gap noted two changes ago, that no ImageLoader was registered at all; its cache sizes are left to Coil deliberately, since they are derived from the device's heap and a number written here would only be a guess.

The same run found a smaller thing that no test could have: opening the search field from inside a library left the scope on "this folder" - the hint read "Search this folder" and a folder toggle sat beside it - while the search in fact ran everywhere, because the view model has always known that a library is not a folder and answered from the index instead. The behaviour was right and the screen was wrong, which is the same shape as the volume-list bug fixed earlier and the same root cause: the switch-on-open asked whether this was the root list rather than whether a folder was open. It now asks for a path, and the scope toggle appears only when there is a folder to choose about, because its two settings did the same thing otherwise. Every other use of isRootView was then checked for the same conflation and none was found. One test was added for the search behaviour; the scope mismatch itself is not testable, being a disagreement between two correct pieces.

324 JVM tests, lint clean. Verified on the phone in both builds: two clips made with ffmpeg (a testsrc2 pattern and SMPTE bars) drew their own patterns as thumbnails, beside a real 4.3 MB video from the device's own storage, and the demo library's three zero-filled stand-in .mp4s fell back to the icon - which exercised the failure path with material that happened to be lying around. The release APK is 2 294 021 bytes and still declares no INTERNET. Not done: the store screenshots still show the home screen from before the library views, and the release build running on a device is in nobody's checklist - which is exactly why the R8 problem survived until now, and is the first thing worth adding.

docs/STORE_LISTING.md

@@ -56,6 +56,15 @@ thumbnails have something to show), a handful of documents, sized stand-ins for
 audio so the storage-usage screen has something worth ranking, and a real ZIP so the extract
 action can be exercised. Every name and every pixel is fake by construction.
 
+One consequence worth knowing before shooting the Videos library: the video stand-ins are
+**zero-filled**, so they cannot be decoded and photograph as the fallback film icon rather than as a
+frame. A screenshot that shows video thumbnails needs a real clip, which is one command:
+
+```bash
+ffmpeg -f lavfi -i "testsrc2=duration=3:size=320x240:rate=15" -pix_fmt yuv420p clip.mp4   # synthetic, no camera needed
+adb push clip.mp4 /sdcard/Download/PlainFiles-Demo/Video/
+```
+
 | Asset | Play's spec | Result | Source |
 |---|---|---|---|
 | App icon | 512×512, 32-bit PNG with alpha, ≤ 1 MB | `store-assets/icon-512.png` — 512×512, **RGBA**, ~5 KB | `tools/store-assets/icon.svg`, mirroring `ic_launcher_background.xml` + `ic_launcher_foreground.xml`. Background, paths and opacities are identical, so **the store icon is the launcher icon** rather than a lookalike that drifts. |