feat(branding): multi-tenant brand naming + per-port email shell + auth UI continuity

Removes the last hardcoded "Port Nimara" references so a tenant cloning
the deploy with a fresh slug sees their own brand throughout.

Browser + native chrome:
- `generateMetadata` reads `branding_app_name` from the first port row
  so the browser tab title, apple-web-app title, and template literal
  reflect the tenant (fallback "CRM" until DB is seeded).
- Mobile topbar derives the brand-mark initials from the port slug
  ("port-nimara" → "PN", "marina-alpha" → "MA") — no code edit on clone.
- `documenso-payload` default redirect URL is `""` so Documenso falls
  back to its own post-sign page instead of routing every tenant's
  signers to portnimara.com; per-port `redirectUrl` setting still wins.
- Server-startup log uses generic "CRM server listening".

Email + auth shell:
- New `auth-shell-branding.ts` resolves logo / background / appName once
  per request from `system_settings`; used by both the email shell and
  the auth-pages SSR layout.
- `auth-branding-provider` wraps `/login`, `/reset-password`, `/set-password`,
  portal `/portal/*` so the branded shell hydrates with the same assets
  the inbox sees.
- `me/email` change email uses the branded shell instead of inline HTML
  with "Port Nimara CRM" baked into copy.
- Admin branding page adds an email-preview card (POSTs to
  `/api/v1/admin/branding/email-preview`) so an admin can spot-check
  their templates before going live.
- `/api/public/files/[id]` exposes branding-category files anonymously
  so inbox images (no session cookie) can render; any other category
  still flows through authenticated `/api/v1/files/[id]/preview`.

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

src/app/(dashboard)/[portSlug]/admin/branding/page.tsx

@@ -4,6 +4,7 @@ import {
 } from '@/components/admin/shared/settings-form-card';
 import { PageHeader } from '@/components/shared/page-header';
 import { PdfLogoUploader } from '@/components/admin/branding/pdf-logo-uploader';
+import { EmailPreviewCard } from '@/components/admin/branding/email-preview-card';
 
 const DEFAULT_EMAIL_HEADER_HTML = `<!-- Optional pre-body header -->
 <table role="presentation" width="100%" cellpadding="0" cellspacing="0" style="border-collapse:collapse;">
@@ -49,7 +50,7 @@ const FIELDS: SettingFieldDef[] = [
     key: 'branding_email_background_url',
     label: 'Email background image',
     description:
-      'Optional blurred photo shown behind the white email card. Leave blank to use the built-in Port Nimara overhead. Recommended: 1920x1080 JPG, pre-blurred to ~20px gaussian so it reads as a soft background even on small clients.',
+      'Blurred photo shown behind the white email card and the auth-shell (login / reset password) pages. Leave blank to render a plain off-white backdrop. Recommended: 1920x1080 JPG, pre-blurred to ~20px gaussian so it reads as a soft background even on small clients.',
     type: 'image-upload',
     defaultValue: '',
   },
@@ -96,6 +97,7 @@ export default function BrandingSettingsPage() {
         description="HTML fragments rendered around every transactional email."
         fields={FIELDS.slice(3)}
       />
+      <EmailPreviewCard />
       <PdfLogoUploader />
     </div>
   );