T08c: Frontend Mail UI + Global Search UI — 44 tests, tsc clean, vite build pass

- Mail page: 3-pane layout (folder tree + mail list + reading pane)
- Compose modal: rich text editor (bold/italic/link), template picker, reply/forward pre-fill
- Mail settings: accounts, signatures, rules, labels, vacation, PGP (6 tabs)
- Shared mailbox selector: switch between personal + shared accounts
- Mail search bar + attachment download + create-event-from-mail
- Global search: tabs for companies/contacts/mails/files/events
- Search autocomplete in TopBar (existing SearchDropdown)
- API client: mail.ts (all endpoints)
- Routes: /mail, /mail/settings
- i18n: de.json + en.json mail + search translations
- 44 new tests (4 test files), full regression 318/318 pass
- tsc --noEmit: 0 errors, vite build: 267 modules

frontend/src/components/mail/SharedMailboxSelector.tsx

@@ -0,0 +1,34 @@
+/**
+ * Shared mailbox selector — switch between personal and shared accounts.
+ */
+
+import React from 'react';
+import { useTranslation } from 'react-i18next';
+import { Select } from '@/components/ui/Select';
+import type { MailAccount } from '@/api/mail';
+
+export interface SharedMailboxSelectorProps {
+  accounts: MailAccount[];
+  selectedAccountId: string;
+  onSelect: (accountId: string) => void;
+}
+
+export function SharedMailboxSelector({ accounts, selectedAccountId, onSelect }: SharedMailboxSelectorProps) {
+  const { t } = useTranslation();
+
+  const options = accounts.map((acc) => ({
+    value: acc.id,
+    label: `${acc.display_name} (${acc.email})${acc.is_shared ? ' — ' + t('mail.shared') : ''}`,
+  }));
+
+  return (
+    <div data-testid="shared-mailbox-selector">
+      <Select
+        label={t('mail.selectAccount')}
+        value={selectedAccountId}
+        onChange={(e) => onSelect(e.target.value)}
+        options={options}
+      />
+    </div>
+  );
+}