fix: use m3ta-home.paths.srcRoot for wallpapers (pure eval compatible)

modules/paths.nix

@@ -0,0 +1,13 @@
+# modules/paths.nix — Provides m3ta-home source paths for pure evaluation mode.
+#
+# Relative paths with ../.. break in the Nix store during pure evaluation.
+# This module exposes the flake source root so other modules can reference
+# assets via config.m3ta-home.paths.srcRoot.
+{lib, ...}: {
+  options.m3ta-home.paths = {
+    srcRoot = lib.mkOption {
+      type = lib.types.path;
+      description = "Root path of the m3ta-home flake source (set by flake.nix).";
+    };
+  };
+}