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.

electron-builder.yml

@@ -0,0 +1,49 @@
+# electron-builder config — LearnHarness desktop (macOS arm64 first).
+#
+# Shape (fact-checked 2026-09-17):
+#   - the app's real server (.next/standalone, curated by
+#     scripts/build-server.mjs) ships via extraResources — OUTSIDE the asar,
+#     because native .node/dylib can't dlopen from a read-only archive and
+#     server.js process.chdir()s into its own dir;
+#   - the asar holds only the shell (electron/main.cjs + package.json);
+#   - signing/notarization config is laid now but inert until a Developer
+#     ID certificate exists (identity: null = unsigned local build, which
+#     runs fine for personal use — Gatekeeper only gates quarantined apps).
+appId: com.learnharness.LearnHarness
+productName: LearnHarness
+directories:
+  output: dist
+files:
+  # Only the shell goes into the asar.
+  - electron/**/*
+  - package.json
+extraResources:
+  # Copy from the PARENT dir (.next), not .next/standalone itself:
+  # electron-builder hard-excludes a copy's root node_modules and the
+  # default no-filter pattern is a literal path match (empirically both
+  # node_modules/ and .next/ were silently dropped → server died on
+  # "Cannot find module 'next'"). With `from: .next` the filter's
+  # relative paths start with standalone/, dodging the root-node_modules
+  # rule, and the explicit wildcard (minimatch dot:true) reaches
+  # standalone/.next and standalone/node_modules.
+  - from: .next
+    to: server
+    filter:
+      - "standalone/**/*"
+mac:
+  target:
+    - target: dmg
+      arch:
+        - arm64
+  category: public.app-category.education
+  # Laid for the STT milestone (researcher-verified): the TCC mic prompt
+  # reads this string; macOS asks regardless of Electron's own handlers.
+  extendInfo:
+    NSMicrophoneUsageDescription: "LearnHarness uses the microphone only for speech-to-text input into your sessions."
+  hardenedRuntime: true
+  entitlements: build/entitlements.mac.plist
+  # Unsigned until a Developer ID certificate exists (research-confirmed:
+  # local builds run fine; distribution needs signing + notarization).
+  identity: null
+dmg:
+  title: "${productName} ${version}"