Add example Thunderbird extension for OAuth2 authentication

docs/thunderbird-oauth-example/README.md

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+# Thunderbird OAuth Extension (Example)
+
+This folder contains a sample `manifest.json` for a Thunderbird OAuth provider extension.
+
+## 1. Update placeholders
+
+Edit [manifest.json](manifest.json) and replace all placeholders:
+
+- `<YOUR_PROVIDER_NAME>`
+- `<your-domain>`
+- `<ISSUER_URL>`
+- `<CLIENT_ID>`
+- `<AUTHORIZATION_ENDPOINT>`
+- `<TOKEN_ENDPOINT>`
+- `<MAIL_DOMAIN>`
+
+## 2. Create the extension package
+
+From this folder, run:
+
+```bash
+zip -r thunderbird-oauth-provider.xpi manifest.json
+```
+
+This creates `thunderbird-oauth-provider.xpi`.
+
+## 3. Install in Thunderbird
+
+1. Open Thunderbird.
+2. Go to **Add-ons and Themes**.
+3. Click the gear icon and choose **Install Add-on From File...**.
+4. Select `thunderbird-oauth-provider.xpi`.
+5. Confirm prompts and restart Thunderbird if asked.
+
+## 4. Verify
+
+- Set account authentication to OAuth2.
+- Trigger sign-in and complete the OAuth login flow.
+- Confirm send/receive works.

docs/thunderbird-oauth-example/manifest.json

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+{
+  "manifest_version": 2,
+  "name": "<YOUR_PROVIDER_NAME>",
+  "version": "1.0.0",
+  "applications": {
+    "gecko": {
+      "id": "oauth-provider@<your-domain>",
+      "strict_min_version": "128.0"
+    }
+  },
+  "oauth_provider": {
+    "issuer": "<ISSUER_URL>",
+    "clientId": "<CLIENT_ID>",
+    "authorizationEndpoint": "<AUTHORIZATION_ENDPOINT>",
+    "tokenEndpoint": "<TOKEN_ENDPOINT>",
+    "redirectionEndpoint": "http://localhost",
+    "usePKCE": true,
+    "hostnames": ["<MAIL_DOMAIN>"],
+    "scopes": "openid email profile"
+  }
+}