chore(desktop-client): add macOS code-signing and notarization scaffolding

- Move the build config out of package.json into electron-builder.yml so
  the mac block can carry hardenedRuntime, entitlements, universal arch,
  and usage-description Info.plist keys. Drop identity:null (which
  forces unsigned builds) and let CSC_NAME or the keychain decide.
- Add entitlements plist files and three helper scripts: sign-setup.sh
  (one-time keychain prep with notarytool store-credentials),
  sign-mac.sh (sign + notarize + staple, supports keychain or CI env
  vars), and ci-import-cert.sh (CI keychain bootstrap).
- Ignore .env.signing so local credentials don't sneak into the repo,
  and pin @emnapi/{core,runtime} as devDependencies so universal builds
  resolve them deterministically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop-client/build/entitlements.mac.inherit.plist

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+  <key>com.apple.security.cs.allow-jit</key><true/>
+  <key>com.apple.security.cs.allow-unsigned-executable-memory</key><true/>
+  <key>com.apple.security.cs.allow-dyld-environment-variables</key><true/>
+  <key>com.apple.security.cs.disable-library-validation</key><true/>
+  <key>com.apple.security.inherit</key><true/>
+</dict>
+</plist>

apps/desktop-client/build/entitlements.mac.plist

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+  <key>com.apple.security.cs.allow-jit</key><true/>
+  <key>com.apple.security.cs.allow-unsigned-executable-memory</key><true/>
+  <key>com.apple.security.cs.allow-dyld-environment-variables</key><true/>
+  <key>com.apple.security.cs.disable-library-validation</key><true/>
+  <key>com.apple.security.device.audio-input</key><true/>
+  <key>com.apple.security.device.camera</key><true/>
+  <key>com.apple.security.network.client</key><true/>
+  <key>com.apple.security.network.server</key><true/>
+  <key>com.apple.security.files.user-selected.read-write</key><true/>
+  <key>com.apple.security.automation.apple-events</key><true/>
+</dict>
+</plist>