feat(send-dialog): surface per-port attachment threshold in preview UI

Per PRE-DEPLOY-PLAN § 1.3.9. Adds an informational banner to the
SendDocumentDialog explaining the size cutoff at which the attachment
switches from inline to a 24h signed-link download. Threshold sourced
from the existing `email_attach_threshold_mb` setting, plumbed through
the previewBody return shape so rep-facing dialogs don't need to call
the admin-only sales-config endpoint.

Bounce monitoring deferred to land alongside the email_bounces table
in Step 3 (schema additions).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/services/document-sends.service.ts

@@ -263,7 +263,16 @@ export async function previewBody(
   recipient: SendRecipientInput,
   customBody: string | null,
   ctx: { berthId?: string; brochureLabel?: string } = {},
-): Promise<{ html: string; markdown: string; unresolved: string[] }> {
+): Promise<{
+  html: string;
+  markdown: string;
+  unresolved: string[];
+  /** Per-port size cutoff at which the attachment is replaced with a
+   * 24-hour signed-link in the email body. Surfaced to the compose UI
+   * so the rep sees up-front whether their attachment will go inline
+   * vs as a link. */
+  attachmentThresholdMb: number;
+}> {
   const content = await getSalesContentConfig(portId);
   const template = customBody?.trim()?.length
     ? customBody
@@ -274,7 +283,12 @@ export async function previewBody(
   const expanded = expandMergeTokens(template, values);
   const unresolved = findUnresolvedTokens(template, values);
   const html = renderEmailBody(expanded);
-  return { html, markdown: expanded, unresolved };
+  return {
+    html,
+    markdown: expanded,
+    unresolved,
+    attachmentThresholdMb: content.emailAttachThresholdMb,
+  };
 }
 
 // ─── Internal helpers ────────────────────────────────────────────────────────