feat(gui): auto-detect games in Wine prefix and browse for exe

- detect::scan_games_in_prefix() walks drive_c/Program Files and
  Program Files (x86) up to depth 4, skipping Windows system dirs,
  the launcher's own exe, and already-configured games
- Empty game list now shows "No games added yet." with two actions:
    · Scan for games — async scan of the Wine prefix, results appear
      as a clickable list with + buttons to add each found exe
    · Browse exe… — native file picker (zenity/kdialog) opening in
      drive_c/, computes the relative path automatically
- Add-game form gains a Browse… button to pick exe from the prefix
- util::pick_file() added alongside pick_folder()

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

src/util.rs

@@ -34,3 +34,30 @@ pub fn pick_folder(title: &str) -> Option<String> {
     }
     None
 }
+
+/// Open a native file picker dialog starting in `start_dir`, or None if
+/// the user cancelled. Tries zenity then kdialog.
+pub fn pick_file(title: &str, start_dir: &str) -> Option<String> {
+    // zenity uses --filename with a trailing slash to open a directory
+    let start_slash = format!("{}/", start_dir.trim_end_matches('/'));
+    let zenity_args = vec![
+        "--file-selection",
+        "--title",
+        title,
+        "--filename",
+        &start_slash,
+    ];
+    let kdialog_args = vec!["--getopenfilename", start_dir, "--title", title];
+    for (cmd, args) in [("zenity", zenity_args.as_slice()), ("kdialog", kdialog_args.as_slice())] {
+        let Ok(out) = std::process::Command::new(cmd).args(args).output() else {
+            continue;
+        };
+        if out.status.success() {
+            let s = String::from_utf8_lossy(&out.stdout).trim().to_string();
+            if !s.is_empty() {
+                return Some(s);
+            }
+        }
+    }
+    None
+}