feat(admin): Phase 4 — doctor, admin completeness, and bubbletea TUI

Adds the admin/diagnostics surface from SPEC §7.2:

- doctor [--account]: per-account IMAP + (RW) SMTP connectivity/auth checks via
  new mail.CheckIMAP/CheckSMTP (connect+auth only, no mail). Exit non-zero on any
  failure; secrets never printed.
- store.UpdateAccount: partial edit, re-encrypts password/secrets only when a
  non-empty value is supplied (blank keeps existing). RecentAuditFor(account).
- config set/get (validates audit_retention_days), audit list [--account][--limit],
  account edit (flag partial-update) / remove [--yes].
- internal/tui: bubbletea AccountForm with pure, fully-tested Fields (validation +
  store.Account assembly + edit prefill). init / bare `account add` / `account edit
  --name X` drop into the TUI; flag forms remain for scripting.

Built test-first; full suite green incl -race. Validated live against the mxlogin
(password) and Gmail (app-password) accounts. Live validation caught a real bug:
doctor authenticated with empty passwords because it iterated ListAccounts (which
strips secrets) — fixed to re-fetch via GetAccount, locked in by a regression test.

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

internal/store/account.go

@@ -100,6 +100,42 @@ func (s *Store) ListAccounts() ([]Account, error) {
 	return out, rows.Err()
 }
 
+// UpdateAccount updates an existing account's mutable fields, matched by Name.
+// The password and OAuth secrets are re-encrypted only when a non-empty value is
+// supplied; a blank value preserves whatever is already stored. Returns
+// ErrAccountNotFound if no account has that name.
+func (s *Store) UpdateAccount(a Account) error {
+	// Build the SET clause, conditionally including secret columns.
+	set := `mode=?, imap_host=?, imap_port=?, imap_security=?,
+	        smtp_host=?, smtp_port=?, smtp_security=?,
+	        auth_type=?, username=?,
+	        whitelist_in_enabled=?, whitelist_out_enabled=?, subject_regex=?, process_backlog=?`
+	args := []any{
+		a.Mode, a.IMAPHost, a.IMAPPort, a.IMAPSecurity,
+		nullStr(a.SMTPHost), nullInt(a.SMTPPort), nullStr(a.SMTPSecurity),
+		a.AuthType, a.Username,
+		b2i(a.WhitelistInEnabled), b2i(a.WhitelistOutEnabled),
+		nullStr(a.SubjectRegex), b2i(a.ProcessBacklog),
+	}
+	if a.Password != "" {
+		enc, err := crypto.Seal(s.key, []byte(a.Password))
+		if err != nil {
+			return err
+		}
+		set += ", enc_password=?"
+		args = append(args, enc)
+	}
+	args = append(args, a.Name)
+	res, err := s.db.Exec("UPDATE accounts SET "+set+" WHERE name=?", args...)
+	if err != nil {
+		return fmt.Errorf("update account: %w", err)
+	}
+	if n, _ := res.RowsAffected(); n == 0 {
+		return ErrAccountNotFound
+	}
+	return nil
+}
+
 func (s *Store) DeleteAccount(name string) error {
 	res, err := s.db.Exec("DELETE FROM accounts WHERE name = ?", name)
 	if err != nil {