Save the config through the shared crate as well

`Config::save` was the same nine lines as dev-updater's, so it is now
`format::write(path, self)`. The reasoning that made those nine lines
correct -- the leftover temp file that keeps its old mode and is then
renamed over the token hashes -- lives with the code and its test rather
than in two places that could stop agreeing.

Verified: 35 tests, clippy silent, rustfmt clean.

server/src/config.rs

@@ -22,7 +22,7 @@ use std::path::{Path, PathBuf};
 use anyhow::{Context, Result};
 use serde::{Deserialize, Serialize};
 
-use wg_app_link::{format, private};
+use wg_app_link::format;
 
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase", default)]
@@ -434,15 +434,7 @@ impl Config {
     /// The mode is set on the temporary file *before* the rename, so the
     /// config is never briefly world-readable at its real path.
     pub fn save(&self, path: &Path) -> Result<()> {
-        if let Some(parent) = path.parent() {
-            private::create_dir(parent)?;
-        }
-        let text = format::render(self).context("serialize config")?;
-        let tmp = path.with_extension("ron.tmp");
-        private::write_file(&tmp, text.as_bytes())?;
-        std::fs::rename(&tmp, path)
-            .with_context(|| format!("replace {} with {}", path.display(), tmp.display()))?;
-        Ok(())
+        format::write(path, self)
     }
 }