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>

README.md

@@ -62,6 +62,7 @@ umutray service install
 | `umutray games [<launcher>]`     | List configured games and their overlay flags           |
 | `umutray diagnose [<name>]`      | Health checks (one launcher or all)                     |
 | `umutray setup <name>`           | Open the graphical setup wizard for a launcher          |
+| `umutray detect [--apply]`       | Scan common Wine prefixes for installed launchers       |
 | `umutray update-proton --latest` | Install newest GE-Proton release                        |
 | `umutray update-proton --list`   | Show recent releases without installing                 |
 | `umutray update-proton`          | Interactive version picker                              |