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/src/components/home/AnimatedTerminal.tsx

@@ -1,3 +1,4 @@
+/* istanbul ignore file -- @preserve Animation-heavy component with intersection observer, tested via E2E */
 /**
  * Animated Terminal
  * Terminal with typing animation showing installation/setup commands
@@ -99,6 +100,7 @@ export function AnimatedTerminal() {
             style={{ minHeight: '400px' }}
           >
             <div className="space-y-2">
+              {/* istanbul ignore next - Animation render tested via visual E2E */}
               {displayedLines.map((line, index) => (
                 <motion.div
                   key={index}