scheme: fix not saving atomically

Causes programs which rely on the save file (e.g. the shell) to fail occasionally as they try to read while the cli is writing

src/caelestia/utils/paths.py

@@ -1,5 +1,8 @@
 import hashlib
+import json
 import os
+import shutil
+import tempfile
 from pathlib import Path
 
 config_dir = Path(os.getenv("XDG_CONFIG_HOME", Path.home() / ".config"))
@@ -31,3 +34,10 @@ def compute_hash(path: str) -> str:
             sha.update(chunk)
 
     return sha.hexdigest()
+
+
+def atomic_dump(path: Path, content: dict[str, any]) -> None:
+    with tempfile.NamedTemporaryFile("w") as f:
+        json.dump(content, f)
+        f.flush()
+        shutil.move(f.name, path)

src/caelestia/utils/scheme.py

@@ -3,7 +3,7 @@ import random
 from pathlib import Path
 
 from caelestia.utils.material import get_colours_for_image
-from caelestia.utils.paths import scheme_data_dir, scheme_path
+from caelestia.utils.paths import atomic_dump, scheme_data_dir, scheme_path
 
 
 class Scheme:
@@ -100,17 +100,16 @@ class Scheme:
 
     def save(self) -> None:
         scheme_path.parent.mkdir(parents=True, exist_ok=True)
-        with scheme_path.open("w") as f:
-            json.dump(
-                {
-                    "name": self.name,
-                    "flavour": self.flavour,
-                    "mode": self.mode,
-                    "variant": self.variant,
-                    "colours": self.colours,
-                },
-                f,
-            )
+        atomic_dump(
+            scheme_path,
+            {
+                "name": self.name,
+                "flavour": self.flavour,
+                "mode": self.mode,
+                "variant": self.variant,
+                "colours": self.colours,
+            },
+        )
 
     def set_random(self) -> None:
         self._name = random.choice(get_scheme_names())