Add genre filter using local appinfo.vdf with API fallback

Adds a second dropdown next to the install filter that lets the user
narrow rolls to a single Steam genre (RPG, Strategy, etc.).

Genre data is loaded from Steam's local appinfo.vdf cache via
python-steam (instant, no network). If that's unavailable or a game
isn't in the cache, falls back to a rate-limited background fetch of
appdetails?filters=genres, prompted via confirm dialog. Cached at
~/.cache/steam-dice/genres.json across runs.

python-steam declared as optdepends — app degrades gracefully to the
API path if missing.

PKGBUILD

@@ -8,6 +8,7 @@ arch=('any')
 url="https://github.com/silvernode/steam-dice"
 license=('GPL-2.0-only')
 depends=('python' 'python-pyqt6' 'python-requests' 'python-keyring' 'xdg-utils')
+optdepends=('python-steam: instant genre filter via Steam'\''s local appinfo.vdf cache (otherwise falls back to rate-limited Steam API)')
 makedepends=('git')
 provides=("$_pkgname")
 conflicts=("$_pkgname")