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.go

@@ -25,11 +25,13 @@ type Mailer interface {
 }
 
 type Deps struct {
-	Store *store.Store
-	Dial  func(store.Account) (Mailer, error)
-	Send  func(store.Account, mail.OutgoingMessage) error
-	Now   func() time.Time
-	Out   io.Writer
+	Store     *store.Store
+	Dial      func(store.Account) (Mailer, error)
+	Send      func(store.Account, mail.OutgoingMessage) error
+	CheckIMAP func(store.Account) error
+	CheckSMTP func(store.Account) error
+	Now       func() time.Time
+	Out       io.Writer
 }
 
 func (d Deps) emit(e Envelope) error {