fix: place Dolphin's User folder a platform sets

Dolphin reads its User folder at $XDG_DATA_HOME/dolphin-emu, else
~/.local/share/dolphin-emu (Source/Core/UICommon/UICommon.cpp:449-477).
The profile names both, so a launcher handing that folder places the
layout:

- EmuELEC starts Dolphin with XDG_DATA_HOME=/storage/roms/dolphin
  (dolphinSA/scripts/dolphin.sh:28): its 30 files move from the BIOS
  root to dolphin/dolphin-emu/ of the install tree.
- RetroDECK links User/Triforce to bios/Triforce
  (dolphin/component_prepare.sh:30, 49), where its list already puts
  segaboot.gcm. The rest of the User folder sits in the Flatpak's data
  directory and is named as read outside the BIOS folder.

An entry only the standalone build reads, path and no standalone_path,
now takes that path as its layout path (Load/Firmware/rtl_bt/), and
the pass copying a name the platform declares elsewhere reaches an
unprefixed path: gba_bios.bin lands in dolphin/dolphin-emu/GBA/.

scripts/common.py

@@ -2985,12 +2985,14 @@ def standalone_layout_path(entry: dict, profile: dict) -> str | None:
 
     ``standalone_path`` when the entry has one. A profile with no libretro
     build has one layout, so its ``path`` is that layout: UAE4All2 reads
-    kickstarts/kick13.rom below its working directory.
+    kickstarts/kick13.rom below its working directory. So does an entry only
+    the standalone build reads (``mode: standalone``), whose destination is
+    its ``path``: Dolphin's Load/Firmware/rtl_bt/ under its User directory.
     """
     own = entry.get("standalone_path")
     if own:
         return own
-    if "libretro" in str(profile.get("type", "")):
+    if "libretro" in str(profile.get("type", "")) and entry.get("mode") != "standalone":
         return None
     return entry.get("path") or None