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]>

docs/linux/apt.md

@@ -0,0 +1,67 @@
+# StepForge on apt-based Linux (Debian / Ubuntu)
+
+This is the setup and packaging guide for **apt-based** distributions. Fedora
+and other dnf-based systems have a separate guide: [dnf.md](dnf.md).
+
+## Install from the .deb
+
+```bash
+sudo apt install ./stepforge_<version>_amd64.deb
+```
+
+apt pulls the required runtime libraries automatically (they are declared as
+`Depends`). The package installs:
+
+- the app and a fixed Electron runtime under `/opt/stepforge`,
+- the `stepforge` launcher at `/usr/bin/stepforge`,
+- a desktop entry, icons, and `.sfgz`/`.sfglt` file associations.
+
+Launch it from your application menu or run `stepforge`.
+
+### Sandbox
+
+The launcher runs **sandboxed**. On most modern kernels the Chromium
+user-namespace sandbox works out of the box; the package's `postinst` also
+makes the setuid `chrome-sandbox` helper usable as a fallback. StepForge will
+**not** silently launch unsandboxed — see the launcher's message if the
+sandbox is unavailable.
+
+## Install from the portable tarball
+
+```bash
+tar -xzf stepforge_<version>_linux-x64.tar.gz
+# Install the runtime libraries first (see below), then run:
+./usr/bin/stepforge         # or move opt/stepforge to /opt and use the launcher
+```
+
+The tarball includes the `/usr/bin/stepforge` launcher (unlike older builds).
+Install the runtime libraries with:
+
+```bash
+bash scripts/linux/apt/install-runtime-deps.sh
+```
+
+## Capture capabilities on apt systems
+
+- **X11**: full per-click capture with an accurate marker (needs `xinput`).
+- **Wayland**: screen capture via the XDG Desktop Portal + PipeWire; the
+  portal asks permission once per recording. Per-click capture with
+  coordinates is not exposed by Wayland, so recording uses a global hotkey or
+  interval trigger. StepForge reports the active trigger honestly.
+
+Run StepForge and open Settings → Diagnostics to see the detected session
+type, portal/PipeWire status, and the active capture profile.
+
+## Build the .deb yourself
+
+```bash
+bash scripts/linux/apt/install-build-deps.sh   # dpkg-dev, fakeroot, xvfb, …
+nvm install && nvm use                          # pinned Node 22 (see .nvmrc)
+npm ci
+npm run package:linux:deb                        # -> build/artifacts/*.deb + tarball + sha256
+```
+
+The builder stages **only** runtime files: the app code, a fixed Electron
+runtime, and production npm dependencies. It never copies the development
+`node_modules`, docs, prompts, or examples, and it fails if `node_modules` is
+missing rather than producing an unusable artifact.