Movie Editor: fix forceInput-STRING blocking generation; surface exact blocker; move core panel up

- Regression fix: making forceInput STRING sockets count as connection inputs made
  auto-wire treat them as required inputs needing a STRING source — with none, the
  builder blocked with 'no source available' while the Requirements panel (model/clip/
  vae/latent/image only) stayed green. forceInput widget sockets always have a widget
  fallback, so connection_inputs now marks them required=False: still wireable, never
  blocking. Resolves 'A required pipeline input has no source' with everything green.
- Generation errors now pass through the builder's 'Generation blocked — <node>.<input>
  (<TYPE>): …' detail instead of a vague generic message, so the exact culprit shows.
- Built-in pipeline panel moved directly under Pipeline requirements so it's findable.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

movie_editor/backend/nodes.py

@@ -116,10 +116,14 @@ def connection_inputs(node_def: dict) -> list[dict]:
             if not isinstance(t, str):
                 continue
             opts = spec[1] if len(spec) > 1 and isinstance(spec[1], dict) else {}
-            # A widget-typed input is a SOCKET (wireable) only when forceInput is set —
-            # e.g. refinement_key_input is ("STRING", {"forceInput": True}). Plain widgets
-            # are handled by widget_inputs, not here.
-            if t in _WIDGET_TYPES and not opts.get("forceInput"):
+            if t in _WIDGET_TYPES:
+                # A widget-typed input is a SOCKET (wireable) only when forceInput is set —
+                # e.g. refinement_key_input is ("STRING", {"forceInput": True}). It always
+                # has a widget fallback, so it's NEVER auto-required (it must not block
+                # generation when left unwired). Plain widgets are handled by widget_inputs.
+                if not opts.get("forceInput"):
+                    continue
+                out.append({"name": name, "type": _normalize_type(t), "required": False})
                 continue
             # V3 nodes can mark a required-group input as optional via the flag.
             is_required = group == "required" and not opts.get("optional", False)

movie_editor/frontend/models.js

@@ -735,9 +735,9 @@
   function body() {
     const b = el("div", "models-body");
     b.append(requirementsPanel());
+    b.append(coreSection());
     b.append(composer());
     b.append(linksSection());
-    b.append(coreSection());
     const v = validation();
     const list = el("div", "slot-list");
     if (!config.slots.length) list.append(el("div", "empty-stage", "No models configured yet — use the form above to add loaders."));

movie_editor/frontend/store.js

@@ -294,12 +294,14 @@
     [/\.vae.*no source/i,        "Missing Video VAE — add a Video VAE loader in Models."],
     [/\.model.*no source/i,      "Missing diffusion model — add a Unet loader in Models."],
     [/\.clip.*no source/i,       "Missing text encoder — add a CLIP loader in Models."],
-    [/no source available/i,     "A required pipeline input has no source — open Models and check the Pipeline requirements checklist."],
     [/not installed/i,           "A configured node class is not installed in ComfyUI — check Models for details."],
     [/Node registry unavailable/i, "ComfyUI is offline or still starting up — wait a moment and try again."],
   ];
 
   function _friendlyGenError(raw) {
+    // "Generation blocked — …" already names the exact input(s); pass it through so the
+    // message is actionable instead of a vague "a required input has no source".
+    if (/Generation blocked/i.test(raw)) return raw;
     for (const [pat, msg] of GEN_ERROR_MAP) if (pat.test(raw)) return msg;
     return raw;
   }