Add read-only external Yomitan profile support

- add `yomitan.externalProfilePath` config and default/template wiring
- load Yomitan from an external Electron profile/session when configured
- disable SubMiner Yomitan writes/settings UI in external-profile mode and update docs/tests

changes/external-yomitan-profile.md

@@ -0,0 +1,5 @@
+type: added
+area: yomitan
+
+- Added `yomitan.externalProfilePath` to reuse another Electron app's Yomitan profile in read-only mode.
+- SubMiner now reuses external Yomitan dictionaries/settings without writing back to that profile.