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>

USER-MANUAL.md

@@ -191,6 +191,7 @@ emcli account add --name work --mode RW \
 | `--smtp-security` | `tls` | `tls` or `starttls` |
 | `--username` | — | Login username, usually your full email (required) |
 | `--password` | — | Login password or app password |
+| `--from` | — | Send-as address (blank = use username); bare or `"Display Name <addr>"` |
 | `--subject-regex` | — | Inbound subject filter (optional) |
 | `--whitelist-in` | off | Enable inbound whitelist |
 | `--whitelist-out` | off | Enable outbound whitelist |