feat: Electron desktop wrapper — LearnHarness.app (macOS arm64)

The web app becomes a desktop app without changing shape: the Electron
shell spawns the Next standalone server on a loopback port and points a
sandboxed window at it. All app logic stays in the Next server.

Bands:

1. LH_DATA_DIR seam (config.ts): PATHS.data/chats/graph derive from
   process.env.LH_DATA_DIR (else repo data/, unchanged in dev). New
   PATHS.modelCache routes the transformers.js embedding-model cache
   (~300MB) to <data>/models when the seam is set — the default cache
   inside node_modules is read-only in a packaged bundle. All existing
   fs writes flow through PATHS; teacherPrompt stays ROOT-anchored
   (read-only asset, ships inside the bundle by design).
   tests/data-dir-seam.test.ts covers set/unset via child process.

2. Standalone build (next.config.ts + scripts/build-server.mjs):
   output 'standalone'; outputFileTracingExcludes data/**/* keeps the
   repo's real session data out of the bundle (verified: an unexcluded
   scratch build traced 34 files / 716K of real chats + ledgers).
   Two empirically-verified tracer gaps required outputFileTracingIncludes:
   pi-ai's provider catalog (JSON import attributes — 39 files, 500s on
   /api/models without them) and onnxruntime-node's sibling dylib
   (libonnxruntime.dlopen via @rpath). build-server.mjs copies public/
   and .next/static per docs, refuses a leaked data/ dir, and asserts
   the runtime-read prompt .md assets are traced.

3. Electron shell (electron/main.cjs, no preload):
   - single-instance lock (two instances would fight over the data dir);
   - free port via listen(0); spawn via process.execPath +
     ELECTRON_RUN_AS_NODE=1 with PORT/HOSTNAME=127.0.0.1/LH_DATA_DIR;
   - BYOK: userData/auth.json, flat {VAR: value} JSON, mode 0600,
     merged into the child env only when absent from the real env
     (auth UI is the settings milestone; file is hand-editable);
   - poll until the server answers (no ready event), then open a
     contextIsolation+sandbox window; deny-by-default permission
     handler (media allowed only from the loopback origin — mic/STT
     groundwork); will-navigate pinned to origin, window.open denied
     with external links to the system browser; macOS Edit-menu roles;
   - quit path SIGTERM→3s→SIGKILL + exit/SIGHUP nets; child stdout
     mirrored to userData/logs/main.log;
   - dev mode via LH_DEV_URL: no spawn, points at next dev (HMR).

4. Packaging (electron-builder.yml + build/entitlements.mac.plist):
   files = electron shell only; the whole .next payload ships via
   extraResources from .next (filter standalone/**/*) — NOT from
   .next/standalone directly: electron-builder hard-excludes a copy's
   root node_modules and the default pattern is a literal match, so
   node_modules/ and .next/ were silently dropped and the server died
   with 'Cannot find module next'. Asar holds no native code.
   Unsigned (identity: null) until a Developer ID cert exists; hardened
   runtime + audio-input entitlements laid now for the STT milestone.
   desktop:dev / desktop:dist scripts (electron 44, electron-builder 26).

Live smoke of the packaged .app: server up on a random loopback port,
all API routes 200, a real GLM chat turn answered end-to-end via
auth.json BYOK, chat persisted under userData/data, second instance
exited via the lock, no orphaned server child after quit.

Gates: typecheck, eslint, 500/500 tests (+2 data-dir-seam), build.

scripts/build-server.mjs

@@ -0,0 +1,60 @@
+/**
+ * build-server.mjs — curate the Electron shell's server payload.
+ *
+ * `next build` (output: "standalone") produces .next/standalone/ with
+ * server.js + the traced node_modules subset, but leaves two pieces
+ * outside that the docs require us to copy: public/ and .next/static/.
+ * This script finishes the job and sanity-checks the result — including
+ * the exclusion that keeps the repo's real data/ OUT of the bundle
+ * (see next.config.ts outputFileTracingExcludes).
+ */
+import * as fs from "node:fs";
+import * as path from "node:path";
+
+const ROOT = path.resolve(import.meta.dirname, "..");
+const STANDALONE = path.join(ROOT, ".next", "standalone");
+
+const copyTree = (from, to) => {
+	fs.cpSync(from, to, { recursive: true });
+};
+
+const exists = (p) => fs.existsSync(p);
+
+if (!exists(path.join(STANDALONE, "server.js"))) {
+	console.error("[build-server] .next/standalone/server.js missing — run `next build` first (output: 'standalone').");
+	process.exit(1);
+}
+
+// 1) Static assets: public/ → standalone/public, .next/static → standalone/.next/static
+const staticFrom = path.join(ROOT, ".next", "static");
+const staticTo = path.join(STANDALONE, ".next", "static");
+if (!exists(staticFrom)) {
+	console.error("[build-server] .next/static missing — the build looks incomplete.");
+	process.exit(1);
+}
+copyTree(staticFrom, staticTo);
+if (exists(path.join(ROOT, "public"))) {
+	copyTree(path.join(ROOT, "public"), path.join(STANDALONE, "public"));
+}
+
+// 2) Safety: the repo's real data/ must NEVER ride along in the bundle.
+const leakedData = path.join(STANDALONE, "data");
+if (exists(leakedData)) {
+	console.error("[build-server] REFUSING: standalone contains a data/ directory — repo session data would ship in the app.");
+	console.error("             Check outputFileTracingExcludes in next.config.ts.");
+	process.exit(1);
+}
+
+// 3) Sanity: the runtime-read prompt assets must be present (the teacher
+//    agent reads them from disk at request time).
+for (const asset of [
+	path.join("src", "agent", "system-prompt.md"),
+	path.join("src", "agent", "prompts", "teacher", "20-teaching.md"),
+]) {
+	if (!exists(path.join(STANDALONE, asset))) {
+		console.error(`[build-server] prompt asset missing from the trace: ${asset}`);
+		process.exit(1);
+	}
+}
+
+console.log("[build-server] server payload ready:", STANDALONE);