Add code signing config for Windows (Azure Artifact Signing) and macOS (Apple notarization)

CI workflows now support code signing when secrets are configured:
- macOS: Apple Developer certificate + App Store Connect API key for notarization
- Windows: Azure Artifact Signing via signtool + dlib
- Both are no-ops when secrets aren't set (backwards-compatible)
- Add Entitlements.plist (mic, network) and Info.plist (NSMicrophoneUsageDescription)
- Add SIGNING.md with full setup guide for both platforms

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/Info.plist

@@ -0,0 +1,8 @@
+<?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>NSMicrophoneUsageDescription</key>
+    <string>Local Transcription needs microphone access for real-time speech-to-text transcription.</string>
+</dict>
+</plist>