feat(ops/campaigns): group merge tags by category + add toolbar hint

Improvements to the variable insertion UX in the Unlayer editor:

1. Reorganized mergeTags from a flat object into 3 logical groups so
   Unlayer's "Merge Tags" dropdown shows them under sub-headers
   instead of a long flat list:

     • Client (firstname, lastname, email, description)
     • Offre (amount, gift_url, expiry, commitment_months)
     • Système (year)

   Format switched from { id: {name, value} } to grouped array
   format (Unlayer accepts both, but groups give better UX).

2. Added `sample` field to each merge tag — Unlayer renders these
   as the visible content while editing, so the canvas shows
   "Louis Tremblay" / "60 $" / "https://gft.link/abc" instead of
   literal "{{firstname}} {{lastname}}". Makes the live preview
   look like real content during edit. Substitution still happens
   server-side at send time via Mustache.

3. New toolbar hint button (code icon, grey) explaining where to
   find merge tags in the Unlayer UI:
   "Insertion : clic dans un texte → barre flottante → icône {}
   Merge Tags. Marche aussi dans les champs URL (boutons, images,
   mailto)."

   This addresses a common discoverability issue: users don't
   always realize variables work in URL fields too (e.g. setting
   a button's "Action URL" to {{gift_url}} so each recipient gets
   their own Giftbit link).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/ops/src/modules/campaigns/pages/TemplateEditorPage.vue

@@ -10,6 +10,13 @@
       <q-chip v-if="lastSavedTs" dense size="sm" color="grey-2" text-color="grey-9" class="q-ml-sm" icon="cloud_done">
         Sauvegardé · {{ lastSavedLabel }}
       </q-chip>
+      <q-btn flat dense icon="code" class="q-ml-sm" color="grey-7">
+        <q-tooltip max-width="320px">
+          <strong>9 variables disponibles</strong> (Client / Offre / Système).
+          Insertion : clic dans un texte → barre flottante → icône <code>{}</code> Merge Tags.
+          Marche aussi dans les champs URL (boutons, images, mailto).
+        </q-tooltip>
+      </q-btn>
       <q-space />
       <q-btn flat color="primary" icon="visibility" label="Aperçu inbox" class="q-mr-sm" @click="openPreview">
         <q-tooltip>Voir le HTML rendu (substitué) tel que reçu par le destinataire</q-tooltip>
@@ -116,17 +123,38 @@ const editorOptions = {
       tools: { dock: 'left' },
     },
   },
-  mergeTags: {
-    firstname:         { name: 'Prénom',          value: '{{firstname}}' },
-    lastname:          { name: 'Nom',             value: '{{lastname}}' },
-    email:             { name: 'Courriel',        value: '{{email}}' },
-    amount:            { name: 'Montant',         value: '{{amount}}' },
-    gift_url:          { name: 'Lien cadeau',     value: '{{gift_url}}' },
-    description:       { name: 'Adresse service', value: '{{description}}' },
-    expiry:            { name: "Date d'expiry",   value: '{{expiry}}' },
-    commitment_months: { name: 'Engagement (mois)', value: '{{commitment_months}}' },
-    year:              { name: 'Année',           value: '{{year}}' },
-  },
+  // Merge tags organized by category — Unlayer shows these in a dropdown
+  // when editing a text block (click into text → toolbar → {} icon) and
+  // ALSO in URL fields (Button "Action URL", Image "Source", mailto links).
+  // The `sample` field is what Unlayer shows as a preview (so the user sees
+  // realistic content while editing); on send, the hub's Mustache renderer
+  // substitutes the actual value.
+  mergeTags: [
+    {
+      name: 'Client',
+      mergeTags: [
+        { name: 'Prénom',          value: '{{firstname}}',   sample: 'Louis' },
+        { name: 'Nom de famille',  value: '{{lastname}}',    sample: 'Tremblay' },
+        { name: 'Courriel',        value: '{{email}}',       sample: 'louis@targo.ca' },
+        { name: 'Adresse service', value: '{{description}}', sample: '123 Rue de la Rivière, Ste-Clotilde' },
+      ],
+    },
+    {
+      name: 'Offre',
+      mergeTags: [
+        { name: 'Montant',           value: '{{amount}}',            sample: '60 $' },
+        { name: 'Lien cadeau (URL)', value: '{{gift_url}}',          sample: 'https://gft.link/abc' },
+        { name: "Date d'expiration", value: '{{expiry}}',            sample: '31 décembre 2026' },
+        { name: 'Engagement (mois)', value: '{{commitment_months}}', sample: '3' },
+      ],
+    },
+    {
+      name: 'Système',
+      mergeTags: [
+        { name: 'Année courante', value: '{{year}}', sample: '2026' },
+      ],
+    },
+  ],
   // Display mode: 'email' (default, with mobile preview), 'web' for landing pages
   displayMode: 'email',
   // Locale for built-in strings