Initial snapshot before step 6 options.js split

docs/EXTENSION_ID.md

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+# Stable Extension ID
+
+Chrome and Brave derive an extension ID from the extension public key. If the
+key changes, the extension ID changes too.
+
+Why this matters for rclone-jav:
+
+- `chrome.storage.sync` settings are scoped to the extension ID.
+- The native messaging host manifest authorizes specific extension IDs in
+  `allowed_origins`.
+- Moving or reloading an unpacked extension without a fixed key can make settings
+  appear to vanish and can break native messaging.
+
+## Recommended workflow
+
+1. Export settings from Options -> Backup before changing install paths.
+2. Keep Production loaded from a stable folder path.
+3. For a permanently stable unpacked ID, add a fixed Chrome extension public key
+   to `manifest.json` under the `key` field.
+4. After the ID changes intentionally, update the native host manifest
+   `allowed_origins` and re-register the host.
+5. Import the settings backup.
+
+## Packed extension workflow
+
+If you pack the extension as a `.crx`, save the generated `.pem` private key.
+Packing future versions with the same `.pem` produces the same extension ID.
+
+Do not lose or share the `.pem`; it controls the extension identity.
+
+## Unpacked extension workflow
+
+For unpacked installs, use the public key derived from the `.pem` as
+`manifest.json`'s `key` value. The value must be the Chrome extension public key
+format, not an arbitrary string.
+
+Same manifest key = same extension ID.