feat(electron): walking skeleton for the desktop shell

Phase 1 step 1 of VNCprodbuild: electron/main.ts boots the same Next.js
"standalone" server artifact the Dockerfile already produces (next.config.ts's
output: "standalone") as a child process on a random localhost port, then
opens a BrowserWindow at it. electron/preload.ts is a contextBridge stub
(window.vnc.isElectron) for now.

scripts/assemble-standalone.mjs copies public/ and .next/static into
.next/standalone, mirroring what the Dockerfile does by hand, since `next
build` deliberately leaves both out of the standalone output.
scripts/build-electron.mjs bundles main.ts/preload.ts to CommonJS via esbuild
(already a devDependency).

New npm scripts: build:standalone, build:electron, electron:dev.
electron-builder.config.js is intentionally minimal - no signing, no
platform targets yet, just enough to prove the concept end to end.

Also fixes a pre-existing repo-wide lint gap: vnc/plugins/smime is an
independent sub-package (own package.json/esbuild build, browser-only
globals) that was never added to eslint's ignores alongside repos:: and
examples/**, so `npm run lint` - and the husky pre-commit hook - was failing
on every commit regardless of what changed. Excluded it the same way those
are, and added node globals for scripts/**/*.mjs so the new build helpers
above lint cleanly too.

Verified manually: npm run build:standalone && npm run build:electron &&
electron . boots the server and opens a window with no errors.

electron-builder.config.js

@@ -0,0 +1,23 @@
+// Base electron-builder config - Phase 1 step 1 of the VNCprodbuild rollout.
+// No code signing and no platform targets configured yet; this exists only
+// to prove the packaging concept end to end (the app runs, boots its own
+// server, opens a window). Targets (dmg/zip/nsis/AppImage/deb), branding
+// icons, and code signing are wired up in later steps - see
+// ~/.claude/skills/VNCprodbuild/SKILL.md, Phase 1 steps 6-9.
+module.exports = {
+  appId: "de.vnc.vncmailplus",
+  productName: "VNCmail+",
+  directories: {
+    output: "dist-electron-builds",
+  },
+  files: ["dist-electron/**/*", "package.json"],
+  extraResources: [
+    {
+      // Same artifact the Dockerfile bakes into the container image (see
+      // Dockerfile + scripts/assemble-standalone.mjs). electron/main.ts
+      // reads it from process.resourcesPath in packaged builds.
+      from: ".next/standalone",
+      to: "standalone",
+    },
+  ],
+};