fix: resolve all 14 ESLint warnings across the codebase

- Add varsIgnorePattern to no-unused-vars for destructuring patterns
- Fix all react-hooks/exhaustive-deps by adding missing dependencies
- Refactor useAnimatedNumber to use ref instead of stale state closure
- Wrap handleLinkResult in useCallback for stable deps
- Extract OAuth utilities from OAuthCallback.tsx to utils/oauth.ts
- Extract background config utilities to utils/backgroundConfig.ts
- Remove unused catch parameter in GiftSubscription

src/utils/oauth.ts

@@ -0,0 +1,48 @@
+export const LINK_OAUTH_STATE_KEY = 'link_oauth_state';
+export const LINK_OAUTH_PROVIDER_KEY = 'link_oauth_provider';
+
+const OAUTH_STATE_KEY = 'oauth_state';
+const OAUTH_PROVIDER_KEY = 'oauth_provider';
+
+export function saveOAuthState(state: string, provider: string): void {
+  sessionStorage.setItem(OAUTH_STATE_KEY, state);
+  sessionStorage.setItem(OAUTH_PROVIDER_KEY, provider);
+}
+
+export function loadOAuthState(): { state: string; provider: string } | null {
+  const state = sessionStorage.getItem(OAUTH_STATE_KEY);
+  const provider = sessionStorage.getItem(OAUTH_PROVIDER_KEY);
+  if (!state || !provider) return null;
+  return { state, provider };
+}
+
+export function clearOAuthState(): void {
+  sessionStorage.removeItem(OAUTH_STATE_KEY);
+  sessionStorage.removeItem(OAUTH_PROVIDER_KEY);
+}
+
+export function peekLinkOAuthState(): { state: string; provider: string } | null {
+  const state = sessionStorage.getItem(LINK_OAUTH_STATE_KEY);
+  const provider = sessionStorage.getItem(LINK_OAUTH_PROVIDER_KEY);
+  if (!state || !provider) return null;
+  return { state, provider };
+}
+
+export function clearLinkOAuthState(): void {
+  sessionStorage.removeItem(LINK_OAUTH_STATE_KEY);
+  sessionStorage.removeItem(LINK_OAUTH_PROVIDER_KEY);
+}
+
+export function getErrorDetail(err: unknown): string | null {
+  if (err && typeof err === 'object' && 'response' in err) {
+    const resp = (err as { response?: { data?: { detail?: unknown } } }).response;
+    const detail = resp?.data?.detail;
+    if (typeof detail === 'string') return detail;
+    if (detail && typeof detail === 'object' && 'message' in detail) {
+      const msg = (detail as Record<string, unknown>).message;
+      if (typeof msg === 'string') return msg;
+    }
+  }
+  if (err instanceof Error) return err.message;
+  return null;
+}