refactor(gecko): derive Dark Reader seed profiles

The seed activations used hand-maintained profile name lists beside programs.firefox.profiles and programs.librewolf.profiles. A future Gecko profile could have received declarative browser config without the writable Dark Reader storage seed.

Deriving the seed list from config.programs.*.profiles keeps the activation target set locked to the actual Home Manager profile attrsets for both Firefox and LibreWolf.

Validation:

nix fmt -- modules/hm-apps/firefox.nix modules/hm-apps/librewolf.nix

nix eval --accept-flake-config --json .#nixosConfigurations.tpnix.config.home-manager.users.vx.programs.firefox.profiles --apply "builtins.attrNames"

nix eval --accept-flake-config --json .#nixosConfigurations.tpnix.config.home-manager.users.vx.programs.librewolf.profiles --apply "builtins.attrNames"

nix eval --accept-flake-config --json .#nixosConfigurations.tpnix.config.home-manager.users.vx.home.activation.seedDarkreaderFirefox.data --apply "builtins.stringLength"

nix eval --accept-flake-config --json .#nixosConfigurations.tpnix.config.home-manager.users.vx.home.activation.seedDarkreaderLibrewolf.data --apply "builtins.stringLength"

nix flake check --accept-flake-config --no-build --offline

chore(nix): update substituters