feat(opencode): sync all providers to live config on directory change

Add additive mode support for OpenCode in sync_current_to_live:
- Add AppType::is_additive_mode() to distinguish switch vs additive mode
- Add AppType::all() iterator to avoid hardcoding app lists
- Add sync_all_providers_to_live() for additive mode apps
- Refactor sync_current_to_live to handle both modes

Frontend changes (directory settings):
- Track opencodeDirChanged in useDirectorySettings
- Trigger syncCurrentProvidersLiveSafe when OpenCode dir changes
- Add i18n strings for OpenCode directory settings

tests/hooks/useSettings.test.tsx

@@ -381,6 +381,7 @@ describe("useSettings hook", () => {
       "/server/claude",
       undefined,
       undefined, // geminiConfigDir
+      undefined, // opencodeConfigDir
     );
     expect(metadataMock.setRequiresRestart).toHaveBeenCalledWith(false);
   });