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.

flake.lock

@@ -98,6 +98,22 @@
         "type": "github"
       }
     },
+    "nix-flatpak": {
+      "locked": {
+        "lastModified": 1767983141,
+        "narHash": "sha256-7ZCulYUD9RmJIDULTRkGLSW1faMpDlPKcbWJLYHoXcs=",
+        "owner": "gmodena",
+        "repo": "nix-flatpak",
+        "rev": "440818969ac2cbd77bfe025e884d0aa528991374",
+        "type": "github"
+      },
+      "original": {
+        "owner": "gmodena",
+        "ref": "latest",
+        "repo": "nix-flatpak",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1776877367,
@@ -141,6 +157,7 @@
       "inputs": {
         "anotherim": "anotherim",
         "lanzaboote": "lanzaboote",
+        "nix-flatpak": "nix-flatpak",
         "nixpkgs": "nixpkgs",
         "sops-nix": "sops-nix"
       }