feat: mail UI improvements - folder mapping, display name, resize perf, list header

- Add configurable folder mapping (sent/drafts/spam/trash) per account
- Migration 0005_folder_mapping.sql with 4 new columns on mail_accounts
- Backend: send_mail uses account.sent_folder_imap_name if set
- Backend: _build_folder_hierarchy respects folder_mapping for is_standard
- Frontend: MailSettings shows folder mapping editor per account
- Frontend: MailSettings allows editing display_name for existing accounts
- Frontend: MailFolderTree shows display_name || email, removes MailIcon
- Frontend: ResizablePanel isDragging state with contain:strict + pointer-events:none
- Frontend: MailList merges select-all and sort controls into one line

frontend/src/api/mail.ts

@@ -19,6 +19,10 @@ export interface MailAccount {
   smtp_port: number;
   is_shared: boolean;
   is_active: boolean;
+  sent_folder_imap_name?: string | null;
+  drafts_folder_imap_name?: string | null;
+  spam_folder_imap_name?: string | null;
+  trash_folder_imap_name?: string | null;
   created_at?: string | null;
   updated_at?: string | null;
 }
@@ -195,6 +199,10 @@ export interface CreateAccountPayload {
   username?: string;
   password: string;
   is_shared?: boolean;
+  sent_folder_imap_name?: string | null;
+  drafts_folder_imap_name?: string | null;
+  spam_folder_imap_name?: string | null;
+  trash_folder_imap_name?: string | null;
 }
 
 export interface UpdateAccountPayload {
@@ -205,6 +213,10 @@ export interface UpdateAccountPayload {
   smtp_port?: number;
   password?: string;
   is_active?: boolean;
+  sent_folder_imap_name?: string | null;
+  drafts_folder_imap_name?: string | null;
+  spam_folder_imap_name?: string | null;
+  trash_folder_imap_name?: string | null;
 }
 
 export interface CreateFolderPayload {