Migrate config load/save/atomic_write to bread_utils::tomlcfg

load_doc/save_doc/atomic_write's bodies now delegate to
bread_utils::tomlcfg and bread_utils::atomic (path dependency for now, see
the TODO in Cargo.toml) instead of owning the temp-then-rename +
.bak-before-overwrite logic locally. Public function names/signatures in
config/mod.rs are unchanged, so none of the ~10 call sites across
ui/views/*.rs needed touching.

This is the other half of tonight's earlier BOS fix pass: that pass gave
breadhelp/src/config.rs its own byte-for-byte copy of this exact logic
(its own doc comment says "same discipline as bos-settings/src/config/
mod.rs::atomic_write") rather than sharing it — breadhelp's migration
follows in the next commit.

Builds clean; all 11 existing tests pass, including the atomic-write
backup/no-leftover-tmp-file test that now exercises the delegated code.

Cargo.lock

@@ -32,6 +32,7 @@ version = "0.6.3"
 dependencies = [
  "async-channel",
  "bread-theme",
+ "bread-utils",
  "glib",
  "gtk4",
  "serde",
@@ -51,6 +52,16 @@ dependencies = [
  "serde_json",
 ]
 
+[[package]]
+name = "bread-utils"
+version = "0.2.3"
+dependencies = [
+ "dirs",
+ "serde",
+ "serde_json",
+ "toml_edit 0.22.27",
+]
+
 [[package]]
 name = "cairo-rs"
 version = "0.22.0"