feat: add email templates with placeholder variables and composer integration

- Reusable email templates with local storage persistence
- Dynamic placeholder variables ({{recipientName}}, {{date}}, etc.) with auto-fill
- Template manager modal with category filtering and search
- Template picker integrated in composer toolbar (Ctrl+Shift+T)
- Settings tab for template management
- 48 unit tests for template utilities
- i18n support for all 8 languages

lib/template-types.ts

@@ -0,0 +1,31 @@
+export interface EmailTemplate {
+  id: string;
+  name: string;
+  subject: string;
+  body: string;
+  category: string;
+  defaultRecipients?: {
+    to?: string[];
+    cc?: string[];
+    bcc?: string[];
+  };
+  identityId?: string;
+  isFavorite: boolean;
+  createdAt: string;
+  updatedAt: string;
+}
+
+export interface PlaceholderVariable {
+  name: string;
+  value: string;
+}
+
+export const BUILT_IN_PLACEHOLDERS = [
+  'recipient_name',
+  'company',
+  'date',
+  'day_of_week',
+  'sender_name',
+] as const;
+
+export type BuiltInPlaceholder = (typeof BUILT_IN_PLACEHOLDERS)[number];