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/cli/agent_test.go

@@ -18,7 +18,9 @@ type fakeMailer struct {
 	full        map[uint32]mail.Message
 }
 
-func (f *fakeMailer) SelectFolder(string) (uint32, uint32, error) { return f.uidValidity, f.maxUID, nil }
+func (f *fakeMailer) SelectFolder(string) (uint32, uint32, error) {
+	return f.uidValidity, f.maxUID, nil
+}
 func (f *fakeMailer) FetchHeaders(_ string, uids []uint32) ([]mail.Header, error) {
 	if len(uids) == 0 {
 		return f.headers, nil