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)

hardening/nixpaks/modules/desktop-base.nix

@@ -48,15 +48,19 @@
       pulse = lib.mkDefault true;
       pipewire = lib.mkDefault false;
     };
-    bind.rw = with sloth; [
-      (concat' runtimeDir "/doc")
-      # Chrome/Firefox/Telegram/etc. use /tmp for single-instance IPC. Share
-      # this directory across launches of the same app only; app IDs differ.
+    bind.rw =
+      with sloth;
       [
-        (mkdir (concat' runtimeDir "/nixpak-${config.flatpak.appId}-tmp"))
-        "/tmp"
+        # Chrome/Firefox/Telegram/etc. use /tmp for single-instance IPC. Share
+        # this directory across launches of the same app only; app IDs differ.
+        [
+          (mkdir (concat' runtimeDir "/nixpak-${config.flatpak.appId}-tmp"))
+          "/tmp"
+        ]
       ]
-    ];
+      # File chooser results arrive through the document portal; an app
+      # without a chooser gets no view of the exported documents.
+      ++ lib.optional config.portalGroups.fileChooser (concat' runtimeDir "/doc");
     bind.ro = [
       # Reuse the host's NixOS fontconfig instead of maintaining a second font
       # set in every sandbox. Nixpak's own font module generates an isolated
@@ -69,10 +73,6 @@
       ]
       "/etc/localtime"
       "/etc/zoneinfo"
-      (sloth.concat' sloth.xdgConfigHome "/gtk-2.0")
-      (sloth.concat' sloth.xdgConfigHome "/gtk-3.0")
-      (sloth.concat' sloth.xdgConfigHome "/gtk-4.0")
-      (sloth.concat' sloth.xdgConfigHome "/kdeglobals")
       "/sys/devices/platform"
     ]
     ++ lib.optionals config.gpu.enable [
@@ -111,8 +111,9 @@
         pkgs.shared-mime-info
       ];
       XCURSOR_PATH = "${pkgs.bibata-cursors}/share/icons:${pkgs.bibata-cursors}/share/pixmaps";
-      # Host NixOS fontconfig, bind-mounted above.
-      FONTCONFIG_FILE = "/etc/fonts/fonts.conf";
+      # Host NixOS fontconfig, bind-mounted above. A policy whose app builds
+      # its own FHS layer overrides this (see modules/gaming-policy.nix).
+      FONTCONFIG_FILE = lib.mkDefault "/etc/fonts/fonts.conf";
       NIXOS_XDG_OPEN_USE_PORTAL = "1";
     }
     // lib.genAttrs [