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>

SIGNING.md

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+# Code Signing Setup
+
+This document explains how to configure code signing for Local Transcription so that Windows and macOS installers are trusted by the operating system.
+
+## Overview
+
+Without code signing:
+- **Windows**: SmartScreen shows "Windows protected your PC" warnings
+- **macOS**: Gatekeeper blocks the app — "app can't be opened because it is from an unidentified developer"
+
+The CI/CD workflows are configured to sign automatically when the required secrets are present. Without secrets, builds still work — they just produce unsigned installers.
+
+---
+
+## Windows — Azure Artifact Signing
+
+**Cost**: ~$9.99/month (up to 5,000 signatures)
+
+### 1. Create an Azure Account
+
+Sign up at https://azure.microsoft.com if you don't already have one.
+
+### 2. Set Up Artifact Signing
+
+1. In the Azure Portal, search for **Artifact Signing**
+2. Create a new **Artifact Signing Account**
+   - Choose a region (e.g., West US 2) — note this for the endpoint URL
+   - The endpoint will be like `https://wus2.codesigning.azure.net/`
+3. Complete **Identity Verification** (required before you can create certificate profiles)
+4. Create a **Certificate Profile** with type "Public Trust" for code signing
+
+### 3. Create an App Registration (Service Principal)
+
+This allows CI to authenticate to Azure:
+
+1. Go to **Azure Active Directory** > **App registrations** > **New registration**
+2. Name it (e.g., `local-transcription-signing`)
+3. After creation, note the **Application (client) ID** and **Directory (tenant) ID**
+4. Go to **Certificates & secrets** > **New client secret** — note the secret value
+5. Grant the app registration the **Artifact Signing Certificate Profile Signer** role on your Artifact Signing Account
+
+### 4. Add Gitea Secrets
+
+In your Gitea repository, go to **Settings** > **Actions** > **Secrets** and add:
+
+| Secret Name | Value |
+|-------------|-------|
+| `AZURE_CLIENT_ID` | App registration Application (client) ID |
+| `AZURE_CLIENT_SECRET` | App registration client secret value |
+| `AZURE_TENANT_ID` | Directory (tenant) ID |
+| `AZURE_SIGNING_ENDPOINT` | Artifact Signing endpoint URL (e.g., `https://wus2.codesigning.azure.net/`) |
+| `AZURE_SIGNING_ACCOUNT` | Artifact Signing account name |
+| `AZURE_CERT_PROFILE` | Certificate profile name |
+
+---
+
+## macOS — Apple Developer Code Signing + Notarization
+
+**Cost**: $99/year (Apple Developer Program)
+
+### 1. Enroll in the Apple Developer Program
+
+Sign up at https://developer.apple.com/programs/
+
+### 2. Create a Developer ID Certificate
+
+1. Open **Xcode** > **Settings** > **Accounts** > select your team > **Manage Certificates**
+2. Click **+** > **Developer ID Application**
+3. Or create via the Apple Developer portal: **Certificates, Identifiers & Profiles** > **Certificates** > **+** > **Developer ID Application**
+
+### 3. Export the Certificate as .p12
+
+1. Open **Keychain Access**
+2. Find your **Developer ID Application** certificate
+3. Right-click > **Export** > save as `.p12` with a password
+4. Base64-encode it:
+   ```bash
+   base64 -i certificate.p12 | tr -d '\n'
+   ```
+
+### 4. Create an App Store Connect API Key
+
+This is used for notarization (submitting the app to Apple for verification):
+
+1. Go to https://appstoreconnect.apple.com/access/integrations/api
+2. Click **Generate API Key**
+3. Give it a name and **Developer** role (minimum)
+4. Download the `.p8` private key file (you can only download it once)
+5. Note the **Key ID** and **Issuer ID** shown on the page
+
+### 5. Find Your Signing Identity
+
+Your signing identity looks like:
+```
+Developer ID Application: Your Name (TEAMID)
+```
+
+You can find it by running:
+```bash
+security find-identity -v -p codesigning
+```
+
+### 6. Add Gitea Secrets
+
+| Secret Name | Value |
+|-------------|-------|
+| `APPLE_CERTIFICATE` | Base64-encoded .p12 certificate (from step 3) |
+| `APPLE_CERTIFICATE_PASSWORD` | Password used when exporting the .p12 |
+| `APPLE_SIGNING_IDENTITY` | Full identity string (e.g., `Developer ID Application: Your Name (TEAMID)`) |
+| `APPLE_API_KEY` | App Store Connect API Key ID |
+| `APPLE_API_ISSUER` | API issuer UUID |
+| `APPLE_API_KEY_CONTENT` | Full contents of the `.p8` private key file |
+
+---
+
+## Verifying Signing Works
+
+### Trigger a Build
+
+Both build workflows use `workflow_dispatch`, so you can trigger them manually in Gitea:
+
+1. Go to **Actions** > select the workflow > **Run workflow**
+2. Enter the release tag (e.g., `v2.0.15`)
+
+### Check macOS
+
+After installing the `.dmg`, the app should open without any Gatekeeper warnings. You can also verify from the command line:
+
+```bash
+codesign -dv --verbose=4 /Applications/Local\ Transcription.app
+spctl --assess --type execute /Applications/Local\ Transcription.app
+```
+
+### Check Windows
+
+After running the `.msi` or `-setup.exe`, there should be no SmartScreen warning. The installer properties should show your organization name as the publisher.