feat(nixpak): sandbox Steam and Heroic in ~/Gaming (#520)

* feat(nixpak): sandbox Steam and Heroic in ~/Gaming

idols-ai stops preserving ~/.local/share/Steam, ~/.steam, ~/.config/heroic
and their game data, which must be moved into ~/Gaming before deploying.

Assisted-by: DeepSeek Harness (deepseek-flash)

* refactor(nixpak): replace steam-fix.nu with sandboxed protontricks

Assisted-by: DeepSeek Harness (claude-opus-5-5)

* fix(nixpak): hide host documents and GTK config from game sandboxes

Assisted-by: DeepSeek Harness (claude-opus-5-5)

* docs(ai): locate the Steam library under ~/Gaming

Assisted-by: DeepSeek Harness (claude-opus-5-5)

* fix(umu-game): confine writes to ~/Gaming and keep foreign entries

Assisted-by: DeepSeek Harness (claude-opus-5-5)

* fix(nixpak): keep Steam and Heroic in the application launcher

The wrappers only carried bin/, so neither launcher had a desktop entry
after replacing programs.steam.

Assisted-by: DeepSeek Harness (deepseek-flash)

* fix(nixpak): give the game sandboxes a fontconfig that sees CJK fonts

Steam rendered Chinese as tofu: the FHS layers build their own /etc/fonts,
whose config lists only DejaVu and the XDG font directory, so neither the
host fonts nor the FHS /usr/share/fonts were ever scanned. Point
FONTCONFIG_FILE at a small config that includes /usr/share/fonts, and ship
WenQuanYi Zen Hei and Noto Sans CJK in all three FHS layers.

Assisted-by: DeepSeek Harness (deepseek-flash)

* fix(nixpak): grant the game launchers a StatusNotifier tray icon

Steam and Heroic register a tray icon over D-Bus, and the game sandboxes
granted no StatusNotifierWatcher access, so the desktop shell had nothing
to show for them. Enable desktopIntegration.tray for all three launchers
and teach the policy suites to expect it.

Assisted-by: DeepSeek Harness (deepseek-flash)

* fix(nixpak): drop the duplicate host CJK font and tidy the font overrides

Steam reads the fonts inside its own FHS layer, so the system-wide wqy_zenhei
added for it no longer serves any sandbox. Give gaming-policy its own comment
for FONTCONFIG_FILE instead of leaving it under the compat-tool one, make the
desktop-base value a default, and note the SNI tray grant in the README table.

Assisted-by: DeepSeek Harness (deepseek-flash)

modules/nixos/desktop/gaming.nix

@@ -23,35 +23,17 @@ in
     #   Begineer Guide: <https://www.reddit.com/r/linux_gaming/wiki/faq/>
     # ==========================================================================
 
-    # Games installed by Steam works fine on NixOS, no other configuration needed.
-    # https://github.com/NixOS/nixpkgs/blob/master/doc/packages/steam.section.md
-    programs.steam = {
-      # Some location that should be persistent:
-      #   ~/.local/share/Steam - The default Steam install location
-      #   ~/.local/share/Steam/steamapps/common - The default Game install location
-      #   ~/.steam/root        - A symlink to ~/.local/share/Steam
-      #   ~/.steam             - Some Symlinks & user info
-      enable = true;
-      package = pkgs.steam;
-      # https://github.com/Winetricks/winetricks
-      # Whether to enable protontricks, a simple wrapper for running Winetricks commands for Proton-enabled games.
-      protontricks.enable = true;
-      # Whether to enable Load the extest library into Steam, to translate X11 input events to uinput events (e.g. for using Steam Input on Wayland) .
-      extest.enable = true;
-      fontPackages = [
-        pkgs.wqy_zenhei # Need by steam for Chinese
-      ];
-      # DW-Proton (Dawn Winery's Proton fork) carries the anti-cheat and game
-      # patches that mainline Proton/GE-Proton lack, so it is what runs the anime
-      # gacha games (Wuthering Waves, Honkai: Star Rail, Arknights: Endfield, ...);
-      # GE-Proton is the general fallback. Both appear as compatibility tools in
-      # each game's Properties -> Compatibility.
-      # https://dawn.wine/dawn-winery/dwproton
-      extraCompatPackages = [
-        pkgs.dwproton-bin
-        pkgs.proton-ge-bin
-      ];
-    };
+    # Steam runs in a nixpak sandbox (hardening/nixpaks/steam.nix) that the
+    # home manager installs, so `programs.steam` is off: its `package` option
+    # calls `.override` on whatever it is given, which a sandbox script cannot
+    # answer, and it would install a second, unsandboxed steam plus `steam-run`.
+    # What is kept from it: 32-bit graphics, which the game sandboxes mount as
+    # /run/opengl-driver-32, and the controller/VR udev rules. Dropped with it:
+    # `extest` (needs /dev/uinput, which no sandbox gets) and the host
+    # protontricks (it would run Wine on game-writable prefixes; the umu
+    # sandbox carries one instead).
+    hardware.graphics.enable32Bit = true;
+    hardware.steam-hardware.enable = true;
 
     # Expose DW-Proton to umu-run: umu ignores Steam's
     # STEAM_EXTRA_COMPAT_TOOLS_PATHS, and a store path baked into a generated