Let a store bring its own carousel logo

The menu entry borrowed the Ports icon, which is what "give it a theme
folder that already has artwork" buys you. EmulationStation offers nothing
better on its own: a system's logo is resolved through the theme
(SystemData::getProperty("image") -> theme's system/logo, path built from
the system's theme folder), and there is no user-level override anywhere —
not in the settings, not in the ROM folder, not in the binary.

So `menu.logo` installs the file where the themes look. The logo folder is
found rather than assumed: it is the folder holding the most logos of
systems every theme has (ports, snes, nes, …), because themes keep them in
art/logos, _inc/systems/logos, _art/Colorlogos and worse. On the box that
found 7 of 9 themes; the other two do not use ${system.theme} at all, and
there ES falls back to drawing the store's name as text, which is a fine
answer.

Two safeguards, since this writes outside the store's own folder for the
first time: the file is named after `menu.theme`, which now defaults to the
store id so no theme's own artwork can be overwritten, and every path
installed is recorded in state.json — an uninstall removes exactly those,
a pre-existing file of that name is reported and left alone.

Themes that ship with the system image are under /usr/share, and Batocera's
root is a RAM overlay: the copy works but does not survive a reboot. The
sync puts it back, and the log says so, naming batocera-save-overlay.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

config.example.json

@@ -16,7 +16,8 @@
     "menu": {
       "mode": "system",
       "name": null,
-      "theme": "ports",
+      "theme": null,
+      "logo": null,
       "updater": true,
       "labels": {}
     },
@@ -63,4 +64,4 @@
     "timeout": 30,
     "insecure": false
   }
-}
+}