Issue #1200: portable mode, a portable.txt beside the executable keeps settings in LightCraftData

As in PhotoCraft: when portable.txt (or LightCraft.portable) sits next to the
executable, config_dir() is <exe dir>/LightCraftData, so settings, logs and
models stay with a portable copy instead of %APPDATA%. LIGHTCRAFT_CONFIG_DIR
still wins and is used without probing beside the executable. A portable
folder that can't be written logs a warning and the platform folder is used.
The check runs once per process.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/whats-new.md

@@ -2,6 +2,11 @@
 
 ## 2026-10-13
 
+### Portable mode
+- A file named `portable.txt` next to the LightCraft program makes it keep its settings, logs and downloaded
+  models in a `LightCraftData` folder beside it instead of the per-user settings folder (issue #1200), as
+  PhotoCraft does. If that folder can't be written, the usual folder is used and the log says why.
+
 ### Slider drags on large raws
 - Dragging a slider no longer re-renders the edited photo's filmstrip thumbnail on every frame; it follows once you
   let go. On a 64 MP raw the first of those renders decoded the whole file again, which held up the loupe's