tangram: persist dconf files

with this, saved tabs are *actually* saved

hosts/common/programs/tangram.nix

@@ -2,22 +2,53 @@
 # it views each tab as a distinct application, persisted, and where the 'home' button action is specific to each tab.
 # it supports ephemeral tabs, but UX is heavily geared to GCing those as early as possible.
 
-{ pkgs, ... }:
+{ lib, pkgs, ... }:
+let
+  dconfProfile = pkgs.writeTextFile {
+    name = "dconf-tangram-profile";
+    destination = "/etc/dconf/profile/tangram";
+    text = ''
+      user-db:tangram
+      system-db:site
+    '';
+  };
+  initTangramDconf = pkgs.writeShellScript "init-tangram-dconf" ''
+    test -f "$1" || ${lib.getBin pkgs.dconf}/bin/dconf compile "$1" "${pkgs.emptyDirectory}"
+  '';
+in
 {
   sane.programs.tangram = {
-    # XXX(2023/07/08): running on moby without this hack fails, with:
+    # XXX(2023/07/08): running on moby without disabling the webkit sandbox fails, with:
     # - `bwrap: Can't make symlink at /var/run: File exists`
     # see epiphany.nix for more info
     package = pkgs.tangram.overrideAttrs (upstream: {
+      # --set DCONF_PROFILE "${dconfProfile}/etc/dconf/profile/tangram"
       preFixup = ''
         gappsWrapperArgs+=(
           --set WEBKIT_DISABLE_SANDBOX_THIS_IS_DANGEROUS "1"
+          --set DCONF_PROFILE "${dconfProfile}/etc/dconf/profile/tangram"
         );
       '' + (upstream.preFixup or "");
     });
+
     persist.private = [
       ".cache/Tangram"
       ".local/share/Tangram"
+      # dconf achieves atomic writes via `mv`, so a symlink doesn't work
+      # moreover, i have to persist the *whole* directory:
+      # - `user-db:tangram/user` causes a schema failure
+      # - bind-mounting `~/private/.config/dconf/tangram` causes dconf to try a cross-fs `mv`, which fails
+      # - dconf provides no way to specify an alternate ~/.config/dconf dir, except by overriding XDG_CONFIG_HOME
+      # { type = "file"; path = ".config/dconf/tangram"; method = "bind"; }
+      ".config/dconf"
     ];
+
+    fs."private/.config/dconf/tangram".generated = {
+      acl.mode = "0644";
+      command = [
+        "${initTangramDconf}"
+        "/home/colin/private/.config/dconf/tangram"
+      ];
+    };
   };
 }