Adds a spindle workflow that packages the Linux AppImage, on the microvm engine with a NixOS image. steam-run is included because Electron's prebuilt binary cannot run unpatched on NixOS; gcc, pkg-config, openssl and python are there for the node-gyp build that better-sqlite3 needed. Pins electron to an exact version, in the manifest and in electron-builder's own electronVersion, so the packaged runtime cannot drift from the one the build resolved. Squashed from 26 commits on the spindles branch. The iteration those commits record was against a tree that predates apps/desktop/main/datastore.ts moving to node:sqlite, and replaying them individually onto the current main conflicts in apps/desktop/package.json, where the lines they edit no longer exist. The `bash` prefixes they added at two call sites are omitted here; a portable shebang in scripts/timed.sh replaces them in a later commit.