feat(cli): configurable send-as From address (flags, TUI, validation)

- tui.ValidFromAddress: exported validator; blank passes, malformed rejects
- Fields.FromAddress: new field, round-trips through ToAccount/FieldsFromAccount
- Fields.Validate: calls ValidFromAddress before returning nil
- TUI form: from_address fieldDef between username and password
- send.go: From set via acc.SendFrom() instead of acc.Username
- admin.go account add: --from flag with pre-parse validation
- admin.go account edit: --from flag; validate before Visit, apply in Visit
- USER-MANUAL.md: --from flag added to account add flags table

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/cli/admin.go

@@ -45,6 +45,7 @@ func runAccount(args []string, role store.Role, out, errOut io.Writer) int {
 		smtpSec := fs.String("smtp-security", "tls", "tls|starttls")
 		user := fs.String("username", "", "login username")
 		pass := fs.String("password", "", "login password")
+		from := fs.String("from", "", "send-as address (blank = use username)")
 		subj := fs.String("subject-regex", "", "inbound subject filter")
 		wlIn := fs.Bool("whitelist-in", false, "enable inbound whitelist")
 		wlOut := fs.Bool("whitelist-out", false, "enable outbound whitelist")
@@ -56,9 +57,14 @@ func runAccount(args []string, role store.Role, out, errOut io.Writer) int {
 			fmt.Fprintln(errOut, "name, imap-host, and username are required")
 			return 2
 		}
+		if err := tui.ValidFromAddress(*from); err != nil {
+			fmt.Fprintln(errOut, err)
+			return 2
+		}
 		acc := store.Account{
 			Name: *name, Mode: *mode, IMAPHost: *host, IMAPPort: *port, IMAPSecurity: *sec,
 			AuthType: "password", Username: *user, Password: *pass,
+			FromAddress: *from,
 			SubjectRegex: *subj, WhitelistInEnabled: *wlIn, WhitelistOutEnabled: *wlOut,
 			ProcessBacklog: *backlog,
 		}
@@ -85,6 +91,7 @@ func runAccount(args []string, role store.Role, out, errOut io.Writer) int {
 		smtpSec := fs.String("smtp-security", "", "tls|starttls")
 		user := fs.String("username", "", "login username")
 		pass := fs.String("password", "", "login password (blank keeps existing)")
+		from := fs.String("from", "", "send-as address (blank keeps existing)")
 		subj := fs.String("subject-regex", "", "inbound subject filter")
 		if err := fs.Parse(rest); err != nil {
 			return 2
@@ -96,6 +103,10 @@ func runAccount(args []string, role store.Role, out, errOut io.Writer) int {
 		if fs.NFlag() == 1 { // only --name → interactive TUI form, prefilled
 			return editInteractive(st, *name, out, errOut)
 		}
+		if err := tui.ValidFromAddress(*from); err != nil {
+			fmt.Fprintln(errOut, err)
+			return 2
+		}
 		acc, err := st.GetAccount(*name)
 		if err != nil {
 			fmt.Fprintf(errOut, "edit: %v\n", err)
@@ -122,6 +133,8 @@ func runAccount(args []string, role store.Role, out, errOut io.Writer) int {
 				acc.Username = *user
 			case "password":
 				acc.Password = *pass
+			case "from":
+				acc.FromAddress = *from
 			case "subject-regex":
 				acc.SubjectRegex = *subj
 			}