Add detect command to find installed launchers on disk

Scans common Wine prefix locations (~/Games, ~/.wine, Lutris, Bottles,
Heroic) plus any user-supplied --dir paths for each configured
launcher's exe. Reports matches with four markers:

  ✓ already configured at that prefix
  → detected at a different prefix (--apply to update)
  ⚠ multiple prefixes match (ambiguous)
  · not found

--apply writes the new prefix_dir back to config.toml for unambiguous
cases; ambiguous ones are skipped with a note to resolve via
`config edit`. The Setup doc comment is also refreshed since the iced
wizard landed in an earlier commit.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/main.rs

@@ -1,4 +1,5 @@
 mod config;
+mod detect;
 mod diagnose;
 mod launcher;
 mod proton;
@@ -61,12 +62,23 @@ enum Commands {
         launcher: Option<String>,
     },
 
-    /// Print setup instructions for a launcher (automated wizard coming soon)
+    /// Open the graphical setup wizard for a launcher
     Setup {
         /// Launcher name
         name: String,
     },
 
+    /// Scan common Wine prefix locations for installed launchers
+    Detect {
+        /// Additional directory to scan (repeatable)
+        #[arg(long, value_name = "PATH")]
+        dir: Vec<PathBuf>,
+
+        /// Write detected prefix_dirs to config
+        #[arg(long)]
+        apply: bool,
+    },
+
     /// Download and switch GE-Proton versions
     UpdateProton {
         /// Install the latest release automatically
@@ -296,6 +308,10 @@ fn main() -> Result<()> {
             setup::run(&config, l)?;
         }
 
+        Commands::Detect { dir, apply } => {
+            detect::run(&config, &dir, apply)?;
+        }
+
         Commands::UpdateProton { latest, version, list } => {
             proton::run(&config, latest, version, list)?;
         }