Add per-game overlay toggles (gamemode, mangohud, gamescope)

Games live under each Launcher as a Vec<Game>; the launcher itself
never picks up overlays, only games launched through `play` do.

- config: Game struct with gamemode/mangohud/gamescope fields, plus
  add_game / remove_game / set_game_flags methods.
- launcher::play_game wraps the game command as
  `gamescope [args] -- gamemoderun umu-run <exe>` (each layer optional)
  and sets MANGOHUD=1 when enabled.
- CLI: `play`, `games`, `config add-game`, `config remove-game`,
  `config set-game-flags`.
- tray: per-game submenus with Play + checkmark toggles for GameMode /
  MangoHud / Gamescope; toggles persist to disk immediately.

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

README.md

@@ -58,6 +58,8 @@ umutray service install
 | `umutray launchers`              | List configured launchers and their state               |
 | `umutray launch <name>`          | Launch a specific launcher (e.g. `umutray launch epic`) |
 | `umutray kill [<name>]`          | Kill one launcher, or all if no name is given           |
+| `umutray play <launcher> <game>` | Play a game with its configured overlays                |
+| `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 update-proton --latest` | Install newest GE-Proton release                        |
@@ -68,6 +70,9 @@ umutray service install
 | `umutray config set …`           | Update globals (`--proton-version`, `--compat-dir`)     |
 | `umutray config add-launcher …`  | Append a new launcher (needs `--exe-path`)              |
 | `umutray config remove-launcher` | Drop a launcher (prefix on disk is left untouched)      |
+| `umutray config add-game …`      | Attach a game to a launcher (needs `--exe-path`)        |
+| `umutray config remove-game …`   | Drop a game from a launcher                             |
+| `umutray config set-game-flags …`| Per-game overlay toggles: gamemode/mangohud/gamescope   |
 | `umutray service install`        | Write + enable a `systemd --user` unit                  |
 | `umutray service uninstall`      | Stop, disable, and remove the unit                      |
 | `umutray service status`         | `systemctl --user status umutray.service`               |