fix(core,gui): send the release version as the client identity

Every release sent the fixed `external-drive-linux@0.1.0-alpha`. If Proton
ever retires that version, every release fails sign-in at once and no update
can fix it.

- core: APP_VERSION and USER_AGENT follow CARGO_PKG_VERSION
  (`external-drive-linux@<version>-stable`, `proton-drive-linux/<version>`)
- core: `app_version` and `user_agent` in config.json are optional overrides;
  the fixed values older releases stored there read as unset and leave the
  file on the next save, a hand-set value is kept
- gui: the About dialog shows the release, its debug info the identity
  actually sent
- docs: describe the new default and the migration

docs/CONFIGURATION.md

@@ -89,7 +89,11 @@ fix it.
 | `open_with` | object | `xdg-open` | — | See [`open_with`](#open_with) |
 | `prompt` | object | built-in window | — | See [`prompt`](#prompt) |
 
-`app_version` and `user_agent` identify the client to Proton. Leave them alone.
+`app_version` and `user_agent` identify the client to Proton. When the keys are absent, the
+client sends its own release (`external-drive-linux@<version>-stable`, `proton-drive-linux/<version>`).
+Leave them out unless a support case asks for a different value. The fixed
+`external-drive-linux@0.1.0-alpha` and `proton-drive-linux/0.1.0` that releases up to 3.1.0 wrote
+here are treated as absent and removed on the next save.
 
 ### `ignore_patterns`