Replace setup CLI stub with iced-based wizard

Lets the user paste an installer URL or local .exe path, downloads to
a temp file if needed, and runs the installer via umu-run with the
launcher's prefix, gameid, and proton path wired up. On completion,
checks for the expected exe and reports next steps.

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

Cargo.toml

@@ -35,3 +35,6 @@ ksni = "0.2"
 
 # HTTP for GE-Proton GitHub releases API
 reqwest = { version = "0.12", features = ["blocking", "json"] }
+
+# GUI for the setup wizard
+iced = "0.13"