Appease X11 users with niri/wayland specific postfixup wrapper (#212)

* Appease X11 users with niri/wayland specific postfixup wrapper

* Linux: stay on native Wayland under niri in the app, not the Nix wrapper

Move the niri check from the flake's wrapper into apps/solvecraft/src/main.rs
next to the XWayland default, so it applies to every niri user, not only
Nix builds (flake.nix is back to main's, which also clears the conflict
with #210). SOLVECRAFT_WAYLAND=1 still keeps native Wayland, and =0 now
forces XWayland. docs/nix.md notes the trade-off: no file drag-and-drop
on native Wayland.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

---------

Co-authored-by: bherbruck <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/nix.md

@@ -87,6 +87,19 @@ For older builds, including 0.3.0, set Anti-aliasing to 1x and restart when usin
 If that window cannot show the settings dialog, edit only the `msaa` field to `1` in
 `~/.local/share/solvecraft/graphics.json`; preserve any other settings.
 
+## Wayland, XWayland and niri
+
+On Linux the app runs through XWayland whenever an X display is available, because
+winit only supports dragging files from a file manager into the window on X11. On
+niri (detected from `XDG_CURRENT_DESKTOP` with a Wayland socket present) it stays on
+native Wayland instead, since window management such as maximize/restore misbehaves
+under niri's XWayland. The trade-off: **on native Wayland, dropping files onto the
+window does not work**; use File > Open or pass the file on the command line.
+This applies to every build, not only the Nix package.
+
+Override the choice with `SOLVECRAFT_WAYLAND=1` (always native Wayland) or
+`SOLVECRAFT_WAYLAND=0` (XWayland whenever an X display is available).
+
 ## Development shells
 
 `nix develop` supplies runtime libraries and uses the Rust/C toolchain already on