Agent email safety: stage drafts for user approval instead of auto-send

Closes the auto-send hole that let earlier models invent signatures
(e.g. signing 'David' for a user named Felix) and SMTP them to real
recipients before the user could review.

New setting: agent_email_confirm (default True).

When on, the MCP send_email and reply_to_email tools no longer SMTP
directly — they write the composed email to scheduled_emails with a new
status 'agent_draft' (far-future send_at so the scheduled-send poller
ignores them) and return a {pending: true, pending_id, to, subject,
body, message: ...} payload. The model surfaces that to the user.

Backend endpoints to approve / cancel:
- GET    /api/email/pending          → list staged drafts for the owner
- POST   /api/email/pending/{id}/approve → flip status to 'pending' +
                                           backdate send_at so the
                                           existing scheduled-send
                                           poller delivers immediately
- DELETE /api/email/pending/{id}     → status = 'cancelled'

UI:
- Settings / AI Defaults gets a new 'Email Safety' card with the
  toggle, default on.
- Tool descriptions for send_email and reply_to_email now include the
  pending behavior + an explicit 'DO NOT invent a signature, do not
  type a person's name' guardrail.

Pass 2 (next): inline chat card with Send / Discard buttons so the user
doesn't have to type a confirmation reply. Today's prompt + the listing
endpoint give the model a clean path to surface drafts.

static/index.html

@@ -1547,6 +1547,10 @@
                working for anyone who wired it via `manage_settings` /
                settings backup. Re-add this card to surface the toggle
                again once the core experience is faster. -->
+          <div class="admin-card">
+            <h2 style="display:flex;align-items:center;gap:6px;"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" style="margin-right:1px;opacity:0.6;flex-shrink:0"><rect x="2" y="4" width="20" height="16" rx="2"/><polyline points="2 6 12 13 22 6"/></svg>Email Safety<span style="flex:1"></span><label class="admin-switch" title="When on, agent send_email and reply_to_email tools stage a draft for your approval instead of sending immediately."><input type="checkbox" id="set-agentEmailConfirm" checked><span class="admin-slider"></span></label></h2>
+            <div class="admin-toggle-sub" style="margin-bottom:8px">When on, agent <code>send_email</code> / <code>reply_to_email</code> tools stage a draft for your approval (in the chat) instead of SMTPing immediately. Stops models from inventing a signature and sending it to a real recipient before you can review.</div>
+          </div>
         </div>
 
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