Add config and service subcommands

config subcommand — show, path, edit ($EDITOR), and a non-interactive
set that takes --prefix / --compat-dir / --gameid. Lets users retarget
the Wine prefix without hand-editing TOML.

service subcommand — install / uninstall / status for a systemd --user
unit that autostarts the tray. install writes ~/.config/systemd/user/
battlenet-manager.service with ExecStart pointing at the current binary,
then daemon-reloads and enable --now's the unit. uninstall tears it back
down.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/config.rs

@@ -36,12 +36,65 @@ fn home_dir() -> PathBuf {
 }
 
 impl Config {
-    fn config_path() -> Result<PathBuf> {
+    pub fn config_path() -> Result<PathBuf> {
         let dirs = ProjectDirs::from("co.aleshym", "", "battlenet-manager")
             .context("Could not determine config directory")?;
         Ok(dirs.config_dir().join("config.toml"))
     }
 
+    /// Print the config path followed by the serialised current config.
+    pub fn show(&self) -> Result<()> {
+        let path = Self::config_path()?;
+        println!("# {}", path.display());
+        let s = toml::to_string_pretty(self)?;
+        print!("{s}");
+        Ok(())
+    }
+
+    /// Open the config file in $EDITOR (falling back to nano).
+    /// Ensures the file exists first so the editor isn't launched on nothing.
+    pub fn edit() -> Result<()> {
+        let _ = Self::load()?; // writes defaults if missing
+        let path = Self::config_path()?;
+        let editor = std::env::var("EDITOR")
+            .or_else(|_| std::env::var("VISUAL"))
+            .unwrap_or_else(|_| "nano".into());
+        let status = std::process::Command::new(&editor)
+            .arg(&path)
+            .status()
+            .with_context(|| format!("Failed to spawn editor '{editor}'"))?;
+        if !status.success() {
+            anyhow::bail!("Editor '{editor}' exited non-zero");
+        }
+        Ok(())
+    }
+
+    /// Update individual fields non-interactively, then save.
+    pub fn set_fields(
+        &mut self,
+        prefix: Option<PathBuf>,
+        compat_dir: Option<PathBuf>,
+        gameid: Option<String>,
+    ) -> Result<()> {
+        if prefix.is_none() && compat_dir.is_none() && gameid.is_none() {
+            anyhow::bail!("nothing to set — pass at least one of --prefix / --compat-dir / --gameid");
+        }
+        if let Some(p) = prefix {
+            self.prefix_dir = p;
+        }
+        if let Some(p) = compat_dir {
+            self.proton_compat_dir = p;
+        }
+        if let Some(g) = gameid {
+            self.gameid = g;
+        }
+        self.save()?;
+        println!("\x1b[1;32m✓\x1b[0m Config saved.");
+        println!();
+        self.show()?;
+        Ok(())
+    }
+
     /// Load config from disk, creating a default one if it doesn't exist.
     pub fn load() -> Result<Self> {
         let path = Self::config_path()?;