Add production .deb packaging, apt setup, desktop integration, and icons

Phase 3 of the improvement plan (PR 8 of the sequence): the apt/X11 packaging
half of Linux support, in separate Linux-specific files. Replaces the old
scripts/package-linux.sh, which the audit flagged as "not production
packaging" (it copied the whole dev node_modules — including vulnerable build
deps — plus docs/prompts/examples/audit files, hardcoded amd64, declared only
xinput, lacked desktop/icon/MIME integration, and could build without
node_modules).

Production builder (packaging/linux/debian/package.sh):
- Stages ONLY runtime files: app code, a fixed Electron runtime, and the
  production npm deps (enumerated via npm ls --omit=dev). Never copies the
  development node_modules; guards against electron-builder/app-builder-lib
  leaking in. Fails if node_modules is absent instead of shipping an unusable
  artifact.
- Detects architecture (dpkg --print-architecture, x64/arm64) rather than
  hardcoding amd64. Generates DEBIAN/control from control.in with proper
  runtime Depends, real maintainer, and homepage.
- Installs a desktop entry, hicolor icons (16–512), .sfgz/.sfglt MIME
  registration, the launcher, and the license. postinst makes chrome-sandbox
  setuid and refreshes desktop/MIME/icon caches; postrm cleans them.
- Emits a .deb, a portable tarball that now INCLUDES /usr/bin/stepforge (the
  old tarball omitted it), and a sha256 sums file.

Launcher (packaging/linux/common/launcher.sh):
- Runs sandboxed; prefers the user-namespace sandbox, accepts a root-owned
  setuid helper, and otherwise refuses to launch with an actionable message.
  --no-sandbox requires an explicit STEPFORGE_ALLOW_NO_SANDBOX opt-in. Never
  installs anything at runtime.

Setup (separate build vs runtime, apt only):
- scripts/linux/apt/install-runtime-deps.sh (Chromium/Electron libs, X11
  tools, portal/PipeWire) and install-build-deps.sh (dpkg-dev, fakeroot,
  xvfb). Runtime script installs no build tools.

Assets: original StepForge icon — packaging/assets/stepforge.svg plus a
generator (scripts/make-icons.js) that renders the PNG set with the repo's own
rasterizer/PNG writer (no third-party art). npm run icons regenerates them.

Wiring: package.json gains package:linux:deb / package:linux:rpm / icons;
build-release.sh uses the production builder and requires node_modules; README
points at the apt/dnf guides.

Tests: tests/unit/packaging-linux.test.js (structural: files present in their
separate locations, old script gone, valid desktop entry, templated arch +
runtime Depends, launcher gates --no-sandbox, builder requires node_modules
and guards dev-dep leaks, apt build/runtime dep separation, original icon set
generates a valid PNG) runs in the normal suite;
tests/integration/linux/package-deb.test.sh builds a real .deb and asserts the
right files present and the dev tree / build tooling / app docs absent
(honest skip only when dpkg-deb/node_modules are genuinely missing).

Verified locally: 276 unit tests pass; the integration test builds and
validates stepforge_0.3.2_amd64.deb; build-release E2E passes with the new
production package.

Co-Authored-By: Claude Fable 5 <[email protected]>

packaging/linux/common/launcher.sh

@@ -0,0 +1,71 @@
+#!/usr/bin/env sh
+# StepForge launcher installed at /usr/bin/stepforge.
+#
+# Runs the packaged Electron runtime against the installed app at
+# /opt/stepforge. It NEVER installs or repairs anything at runtime and it does
+# NOT silently disable the Chromium sandbox: an unsandboxed launch requires the
+# explicit STEPFORGE_ALLOW_NO_SANDBOX=1 opt-in (development/CI only).
+
+set -eu
+
+APP_DIR=/opt/stepforge
+ELECTRON="$APP_DIR/node_modules/electron/dist/electron"
+SANDBOX_HELPER="$APP_DIR/node_modules/electron/dist/chrome-sandbox"
+
+if [ ! -x "$ELECTRON" ]; then
+  echo "stepforge: Electron runtime missing at $ELECTRON (reinstall the package)." >&2
+  exit 1
+fi
+
+cd "$APP_DIR" || exit 1
+
+# Linux screen capture: enable the PipeWire path for Wayland portals; harmless
+# on X11 where Ozone auto-selects.
+COMMON_ARGS="--enable-features=WebRTCPipeWireCapturer --ozone-platform-hint=auto"
+
+sandbox_ok() {
+  [ -e "$SANDBOX_HELPER" ] || return 1
+  helper_uid="$(stat -c '%u' "$SANDBOX_HELPER" 2>/dev/null || echo '')"
+  helper_mode="$(stat -c '%a' "$SANDBOX_HELPER" 2>/dev/null || echo '')"
+  [ "$helper_uid" = "0" ] || return 1
+  [ -n "$helper_mode" ] || return 1
+  # setuid bit set?
+  [ $(( $((8#$helper_mode)) & 04000 )) -ne 0 ] || return 1
+  return 0
+}
+
+userns_ok() {
+  # Namespaced sandbox works without the setuid helper on kernels that allow
+  # unprivileged user namespaces.
+  if [ -r /proc/sys/kernel/unprivileged_userns_clone ]; then
+    [ "$(cat /proc/sys/kernel/unprivileged_userns_clone)" = "1" ] && return 0 || return 1
+  fi
+  if [ -r /proc/sys/kernel/apparmor_restrict_unprivileged_userns ]; then
+    [ "$(cat /proc/sys/kernel/apparmor_restrict_unprivileged_userns)" = "0" ] && return 0 || return 1
+  fi
+  [ -e /proc/self/ns/user ] && return 0 || return 1
+}
+
+if sandbox_ok || userns_ok; then
+  exec "$ELECTRON" $COMMON_ARGS "$APP_DIR" "$@"
+fi
+
+if [ "${STEPFORGE_ALLOW_NO_SANDBOX:-}" = "1" ] || [ "${ELECTRON_DISABLE_SANDBOX:-}" = "1" ]; then
+  echo "stepforge: launching WITHOUT the Chromium sandbox (explicit opt-in)." >&2
+  exec "$ELECTRON" --no-sandbox $COMMON_ARGS "$APP_DIR" "$@"
+fi
+
+cat >&2 <<'MSG'
+stepforge: the Chromium sandbox is not available and StepForge will not launch
+unsandboxed by default.
+
+Fix one of the following:
+  * Make the setuid sandbox helper usable:
+      sudo chown root:root /opt/stepforge/node_modules/electron/dist/chrome-sandbox
+      sudo chmod 4755 /opt/stepforge/node_modules/electron/dist/chrome-sandbox
+  * Enable unprivileged user namespaces (kernel/sysctl dependent):
+      sudo sysctl -w kernel.unprivileged_userns_clone=1
+
+For development/CI only you may set STEPFORGE_ALLOW_NO_SANDBOX=1 to override.
+MSG
+exit 1

packaging/linux/common/stepforge-mime.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
+  <mime-type type="application/x-stepforge-guide">
+    <comment>StepForge guide archive</comment>
+    <glob pattern="*.sfgz"/>
+    <icon name="stepforge"/>
+  </mime-type>
+  <mime-type type="application/x-stepforge-template">
+    <comment>StepForge export template</comment>
+    <glob pattern="*.sfglt"/>
+    <icon name="stepforge"/>
+  </mime-type>
+</mime-info>

packaging/linux/common/stepforge.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Type=Application
+Name=StepForge
+GenericName=Step-by-step guide capture
+Comment=Capture, annotate, and export step-by-step guides
+Exec=stepforge %U
+Icon=stepforge
+Terminal=false
+Categories=Office;Graphics;Utility;
+Keywords=documentation;screenshot;guide;capture;steps;
+StartupNotify=true
+StartupWMClass=StepForge
+MimeType=application/x-stepforge-guide;