test(compose): add findComposeIdentityId to reply-identity mock

The recipient chip-drag and paste tests render <EmailComposer>, which since
b716f95a (feat(compose): preselect identity of the active mailbox) calls
findComposeIdentityId() from @/lib/reply-identity in compose mode. Both tests
mock that module but only returned resolveReplyFrom, so vitest threw
"No 'findComposeIdentityId' export is defined on the mock" on mount,
failing all 18 tests. Add the missing export (returns null; the composer
guards with if (composeIdentityId)).

components/email/__tests__/recipient-chip-drag.test.tsx

@@ -148,7 +148,10 @@ vi.mock('@/lib/email-sanitization', () => ({
   parseHtmlSafely: (html: string) => new DOMParser().parseFromString(html, 'text/html'),
 }));
 
-vi.mock('@/lib/reply-identity', () => ({ resolveReplyFrom: () => null }));
+vi.mock('@/lib/reply-identity', () => ({
+  resolveReplyFrom: () => null,
+  findComposeIdentityId: () => null,
+}));
 vi.mock('@/lib/email-threading', () => ({
   computeReplyThreadingHeaders: () => ({ inReplyTo: [], references: [] }),
 }));

components/email/__tests__/recipient-paste.test.tsx

@@ -147,7 +147,10 @@ vi.mock('@/lib/email-sanitization', () => ({
   parseHtmlSafely: (html: string) => new DOMParser().parseFromString(html, 'text/html'),
 }));
 
-vi.mock('@/lib/reply-identity', () => ({ resolveReplyFrom: () => null }));
+vi.mock('@/lib/reply-identity', () => ({
+  resolveReplyFrom: () => null,
+  findComposeIdentityId: () => null,
+}));
 vi.mock('@/lib/email-threading', () => ({
   computeReplyThreadingHeaders: () => ({ inReplyTo: [], references: [] }),
 }));