feat(assets): the real Atelier artwork, replacing the amber swatches

Both masters already existed in OfficerIcons — the painter duck with the
beret and palette — so the plugin was showing a flat #f59e0b square for want
of a copy step, not for want of art.

The icon master is 1312x1199, not square, so it is trimmed to the artwork
before it is fitted to 256: resizing that canvas directly would have drawn the
duck at about four fifths the size everything else in the dock uses. Splash is
a straight 1254 -> 1024, and PLACEHOLDER.md goes with them.

PLUGIN.md

@@ -170,8 +170,11 @@ expected, not a bug.
    `BLOCKERS.md` for why and the exact commands.
 2. **`OFFICERDEV_POSTGRES_URL` has to be set before install.** No default, on purpose.
 3. **The `atelier` → `officer_db` migration**, described above, once someone can watch it happen.
-4. **`assets/icon.png` and `assets/splash.png` are `[placeholder]`** flat swatches — see
-   `assets/PLACEHOLDER.md`.
+4. ~~`assets/icon.png` and `assets/splash.png` are `[placeholder]` flat swatches.~~ Done
+   2026-08-20: both come from the suite's own masters in the OfficerIcons repo —
+   `OffAtelierAppIcon` (trimmed, since that master is 1312x1199 rather than square, then centred on
+   a transparent 256 canvas) and `OffAtelierSplashScreen` at 1024. Same shapes and same convention as
+   plugins/music.
 5. **A real PM2 install/enable/disable/uninstall/purge cycle**, on a machine where doing so does not
    touch the owner's live session or shared database.
 6. **Deciding whether and when to cut the mobile app over** from `atelier.officer.dev` to the plugin's