Take XDG resolution from the shared crate too

Sixth and last of the modules that were the link rather than this
product. main.rs loses config_home, data_home and xdg_dir, and its test
module with them -- it held one test, which moved to the crate that now
holds the code.

The helpers gained a `product` parameter, matching certs::ensure and
netif::wg_address, which is what keeps two products' state apart while
resolving it identically.

Verified by running it: with only XDG_CONFIG_HOME and XDG_DATA_HOME set
and no --config or --data-dir, the server puts its certificates in
$XDG_CONFIG_HOME/ai-app/certs and its sessions in
$XDG_DATA_HOME/ai-app/sessions, and still prints an aiapp:// enrollment
URI. 35 tests here and 19 in the crate, clippy silent, rustfmt clean.

server/src/main.rs

@@ -33,40 +33,13 @@ use tokio::signal::unix::{SignalKind, signal};
 
 use wg_app_link::enroll;
 use wg_app_link::netif::{self, WG_INTERFACE};
+use wg_app_link::xdg::{config_home, data_home};
 
 use config::TokenEntry;
 use session::SessionManager;
 
 const DEFAULT_PORT: u16 = 8443;
 
-/// `$XDG_CONFIG_HOME/ai-app`, or `~/.config/ai-app`. Holds `config.ron`
-/// and `certs/`.
-fn config_home() -> PathBuf {
-    xdg_dir(std::env::var_os("XDG_CONFIG_HOME"), ".config")
-}
-
-/// `$XDG_DATA_HOME/ai-app`, or `~/.local/share/ai-app`. Holds the session
-/// directories: transcripts, attachments, produced images.
-fn data_home() -> PathBuf {
-    xdg_dir(std::env::var_os("XDG_DATA_HOME"), ".local/share")
-}
-
-/// This app's directory under `base` -- the XDG variable's value, if it
-/// was set to an absolute path as the spec requires -- or under
-/// `~/<fallback>` otherwise. Takes the value rather than reading the
-/// environment itself so the rule is testable without mutating a
-/// process-wide variable other threads may be reading.
-fn xdg_dir(base: Option<std::ffi::OsString>, fallback: &str) -> PathBuf {
-    base.map(PathBuf::from)
-        .filter(|path| path.is_absolute())
-        .unwrap_or_else(|| {
-            std::env::home_dir()
-                .unwrap_or_else(|| PathBuf::from("."))
-                .join(fallback)
-        })
-        .join("ai-app")
-}
-
 /// Serves AI coding sessions (Claude Code, llama.cpp) to the phone app.
 #[derive(Parser)]
 struct Args {
@@ -121,16 +94,16 @@ async fn main() -> Result<()> {
 
     let config_path = args
         .config
-        .unwrap_or_else(|| config_home().join("config.ron"));
+        .unwrap_or_else(|| config_home("ai-app").join("config.ron"));
     let data_dir = args
         .data_dir
-        .unwrap_or_else(|| data_home().join("sessions"));
+        .unwrap_or_else(|| data_home("ai-app").join("sessions"));
     // Beside the session data rather than under it: models outlive every
     // session and are shared by all of them, so deleting a session must
     // never take a multi-gigabyte download with it.
     let models_dir = args
         .models_dir
-        .unwrap_or_else(|| data_home().join("models"));
+        .unwrap_or_else(|| data_home("ai-app").join("models"));
     let models = Arc::new(models::ModelStore::new(models_dir.clone()));
     let manager = Arc::new(
         SessionManager::new(config_path.clone(), data_dir, models_dir.clone())
@@ -163,7 +136,9 @@ async fn main() -> Result<()> {
     // also what the phone app embeds at build time, so they need to be
     // obtainable on a machine whose tunnel isn't up yet. The leaf is
     // reissued on every start, so once wg0 exists the next start covers it.
-    let certs_dir = args.certs.unwrap_or_else(|| config_home().join("certs"));
+    let certs_dir = args
+        .certs
+        .unwrap_or_else(|| config_home("ai-app").join("certs"));
     let certificates = wg_app_link::certs::ensure("ai-app", &certs_dir, &netif::local_addresses())
         .with_context(|| format!("failed to prepare certificates in {}", certs_dir.display()))?;
     if certificates.ca_is_new {
@@ -247,28 +222,3 @@ async fn main() -> Result<()> {
 
     Ok(())
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn xdg_dirs_respect_the_environment_and_are_namespaced() {
-        let home_fallback = xdg_dir(None, ".config");
-        assert!(home_fallback.ends_with("ai-app"));
-        assert!(home_fallback.parent().expect("parent").ends_with(".config"));
-
-        assert_eq!(
-            xdg_dir(Some("/somewhere".into()), ".config"),
-            PathBuf::from("/somewhere/ai-app"),
-        );
-
-        // Relative values are ignored per the spec, rather than resolving
-        // against whatever the working directory happens to be -- so a
-        // relative setting lands on the same path as no setting at all.
-        assert_eq!(
-            xdg_dir(Some("relative/path".into()), ".config"),
-            home_fallback
-        );
-    }
-}