feat(templates): add pagination with total counts to catalog and campaign picker

Test Catalog's pagination showed only 'Page N' with no way to know
how many pages existed, and the campaign 'Add Test' modal's template
picker had no pagination at all — with 2800+ templates in the
catalog, its flat 100-row fetch made most templates unreachable
unless a search happened to match. Adds GET /test-templates/count
(open to any authenticated user, mirrors the list endpoint's filters)
and wires real 'Page N of M' + total-available counts into both.

frontend/src/api/test-templates.ts

@@ -65,6 +65,25 @@ export async function getTemplates(
   return data;
 }
 
+/** Total count of templates matching the same filters as getTemplates() — for pagination. */
+export async function getTemplateCount(
+  filters?: Omit<TemplateFilters, "offset" | "limit">,
+): Promise<number> {
+  const params = new URLSearchParams();
+  if (filters?.source) params.append("source", filters.source);
+  if (filters?.platform) params.append("platform", filters.platform);
+  if (filters?.severity) params.append("severity", filters.severity);
+  if (filters?.mitre_technique_id)
+    params.append("mitre_technique_id", filters.mitre_technique_id);
+  if (filters?.search) params.append("search", filters.search);
+  params.append("is_active", filters?.is_active !== undefined ? String(filters.is_active) : "true");
+
+  const { data } = await client.get<{ total: number }>(
+    `/test-templates/count${params.toString() ? `?${params}` : ""}`,
+  );
+  return data.total;
+}
+
 // ── Detail ─────────────────────────────────────────────────────────
 
 /** Fetch a single test template by ID. */