Add unit tests for OAuthButtons and LinkedAccountsSettings components

- Introduced comprehensive test coverage for `OAuthButtons` and `LinkedAccountsSettings`, including loading states, button behaviors, error handling, and custom class support.
- Implemented `LinkedAccountsPage` tests for rendering and component integration.
- Adjusted E2E coverage exclusions in various components, focusing on UI-heavy and animation-based flows best suited for E2E tests.
- Refined Jest coverage thresholds to align with improved unit test additions.

frontend/jest.config.js

@@ -43,10 +43,10 @@ const customJestConfig = {
   ],
   coverageThreshold: {
     global: {
-      branches: 85,
-      functions: 85,
-      lines: 90,
-      statements: 90,
+      branches: 90,
+      functions: 97,
+      lines: 97,
+      statements: 97,
     },
   },
 };