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.

src/config/mod.rs

@@ -21,27 +21,12 @@ use toml_edit::{value, Array, DocumentMut, Item, Table, Value};
 /// breadcrumbs' saved network passwords, ...). Back up the unparseable file
 /// once before falling back, so a bad edit is always recoverable.
 pub fn load_doc(path: &Path) -> DocumentMut {
-    let Ok(text) = std::fs::read_to_string(path) else {
-        return DocumentMut::default();
-    };
-    match text.parse::<DocumentMut>() {
-        Ok(doc) => doc,
-        Err(e) => {
-            let backup = PathBuf::from(format!("{}.bak", path.display()));
-            eprintln!(
-                "bos-settings: {} failed to parse ({e}); backed up to {} before falling back to defaults",
-                path.display(),
-                backup.display()
-            );
-            let _ = std::fs::write(&backup, &text);
-            DocumentMut::default()
-        }
-    }
+    bread_utils::tomlcfg::load_doc("bos-settings", path)
 }
 
 /// Write the document back to disk, creating parent dirs as needed.
 pub fn save_doc(path: &Path, doc: &DocumentMut) -> Result<(), Box<dyn Error>> {
-    atomic_write(path, &doc.to_string())?;
+    bread_utils::tomlcfg::save_doc(path, doc)?;
     Ok(())
 }
 
@@ -50,42 +35,16 @@ pub fn save_doc(path: &Path, doc: &DocumentMut) -> Result<(), Box<dyn Error>> {
 ///
 /// Every config-writing view in this app (TOML via `save_doc` above, and the
 /// plain-JSON views — keybinds, autostart, appearance/settings.json,
-/// monitors.json, breadbar's CSS) should go through this instead of a bare
-/// `std::fs::write`: writing straight to the target path means a crash,
-/// power loss, or disk-full error mid-write can leave the file truncated or
-/// corrupted with no way back. Writing to a temp file in the *same*
-/// directory first, then `rename`-ing it over the target, avoids that — a
-/// rename within one filesystem is atomic, so the target either has the old
-/// complete contents or the new complete contents, never a partial write.
-/// Backing up the previous file first (best-effort — the write can still
-/// proceed if the backup fails, e.g. read-only source) means even a
-/// successful-but-wrong write is always recoverable from `<path>.bak`.
+/// monitors.json, breadbar's CSS) goes through this instead of a bare
+/// `std::fs::write` — see `bread_utils::atomic::write_atomic_backed_up`'s
+/// doc comment for why (crash/power-loss safety via temp-then-rename, plus
+/// a `.bak` of whatever was there before).
 pub fn atomic_write(path: &Path, contents: &str) -> std::io::Result<()> {
-    if let Some(parent) = path.parent() {
-        std::fs::create_dir_all(parent)?;
-    }
-    if path.exists() {
-        let backup = PathBuf::from(format!("{}.bak", path.display()));
-        let _ = std::fs::copy(path, &backup);
-    }
-    let dir = path.parent().map(Path::to_path_buf).unwrap_or_else(|| PathBuf::from("."));
-    let file_name = path.file_name().and_then(|n| n.to_str()).unwrap_or("config");
-    let tmp_path = dir.join(format!(".{file_name}.tmp.{}", std::process::id()));
-    std::fs::write(&tmp_path, contents)?;
-    std::fs::rename(&tmp_path, path)?;
-    Ok(())
+    bread_utils::atomic::write_atomic_backed_up(path, contents)
 }
 
 pub fn config_dir() -> PathBuf {
-    // Honour XDG_CONFIG_HOME if set; otherwise fall back to $HOME/.config.
-    if let Ok(xdg) = std::env::var("XDG_CONFIG_HOME") {
-        let p = PathBuf::from(xdg);
-        if p.is_absolute() {
-            return p;
-        }
-    }
-    let home = std::env::var("HOME").unwrap_or_else(|_| "/root".to_string());
-    PathBuf::from(home).join(".config")
+    bread_utils::xdg::config_home()
 }
 
 // --- typed readers (walk a dotted path, return None if absent/wrong type) ---