Say a prompt's role for values kept from pictures

Current node showed "negative prompt" only on values from a workflow. A material's blocks carried no role, and when a picture and a workflow held the same text the merged card kept the picture's empty one, so a negative prompt could be offered to a positive box with nothing to say so.

- api/materials.py works the role out by what the prompt was wired to, as the material details already do; it reads the stored workflow, so pictures kept earlier get it too.
- node_parameter_sets.js takes the role from whichever source knows it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/modules/node_parameter_sets.js

@@ -26,6 +26,8 @@ async function fetchJson(url, signal) {
     return response.json();
 }
 
+const KNOWN_ROLES = new Set(['positive', 'negative', 'both']);
+
 /**
  * [{ name, role, timestamp, values, sources }], newest first. A source is a material
  * { kind: 'material', label, filename, timestamp, origin, image, preview } (origin: how it was
@@ -51,6 +53,7 @@ export async function loadParameterSets(type, { signal, boxes = [] } = {}) {
         else {
             known.sources.push(...entry.sources);
             known.timestamp = Math.max(known.timestamp, entry.timestamp);
+            if (!KNOWN_ROLES.has(known.role)) known.role = entry.role; // one source may not know it
         }
     };
     for (const material of materials.materials || []) {