Add modular dev environment installer, dotfiles, and desnap script

Adds a TUI-driven, rollback-capable installer (install/install.sh +
install/modules/*.sh) covering JS/Python/systems toolchains, dev tooling,
browsers, chat apps, GNOME extras, and misc setup (wakatime key, ssh/gpg
perms, apt mirror benchmarking), all sourced from each vendor's official
install method. bootstrap.sh stows the zsh/git/tmux/vscode dotfiles and
runs the installer; desnap.sh purges snapd and replaces default snaps with
apt/Flatpak equivalents.

install/modules/pipenv.sh

@@ -0,0 +1,12 @@
+install_pipenv() {
+  if ! has_cmd pipx; then
+    err "pipx not found; run the pipx module first"
+    return 1
+  fi
+  if pipx list --short 2>/dev/null | grep -q '^pipenv '; then
+    ok "pipenv already installed"
+    return
+  fi
+  pipx install pipenv
+  rollback_push "pipx uninstall pipenv"
+}