gaming: declarative Flatpak via nix-flatpak (Bolt, Sober), Heroic + Proton-GE

Add nix-flatpak flake input and modules/gaming.nix managing Flatpak
remotes/packages declaratively: Bolt (Jagex launcher for RS3/OSRS) and
Sober (Roblox). Add Heroic and hyperplay-ext, plus Proton-GE for Steam.
Drops the old imperative runescape-launcher.

modules/packages.nix

@@ -14,6 +14,7 @@
     "terraform"
     "packer"
     "obsidian"
+    "hyperplay-ext"
   ];
 
   # List packages installed in system profile.
@@ -66,6 +67,9 @@
     obs-studio
     inputs.anotherim.packages.${pkgs.system}.default
 
+    # Gaming
+    heroic
+
     # Development
     nodejs_20
     gnupg