Improve ProjectLoaderDynamic UX: single node, error feedback, auto-refresh

Remove 3 redundant hardcoded nodes (Standard/VACE/LoRA), keeping only the
Dynamic node. Add total_sequences INT output (slot 0) for loop counting.
Add structured error handling: _fetch_json returns typed error dicts,
load_dynamic raises RuntimeError with descriptive messages, JS shows
red border/title on errors. Add 500ms debounced auto-refresh on widget
changes. Add 404s for missing project/file in API endpoints.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_db.py

@@ -172,6 +172,25 @@ class TestSequences:
         db.delete_sequences_for_file(df_id)
         assert db.list_sequences(df_id) == []
 
+    def test_count_sequences(self, db):
+        pid = db.create_project("p1", "/p1")
+        df_id = db.create_data_file(pid, "batch", "generic")
+        assert db.count_sequences(df_id) == 0
+        db.upsert_sequence(df_id, 1, {"a": 1})
+        db.upsert_sequence(df_id, 2, {"b": 2})
+        db.upsert_sequence(df_id, 3, {"c": 3})
+        assert db.count_sequences(df_id) == 3
+
+    def test_query_total_sequences(self, db):
+        pid = db.create_project("p1", "/p1")
+        df_id = db.create_data_file(pid, "batch", "generic")
+        db.upsert_sequence(df_id, 1, {"a": 1})
+        db.upsert_sequence(df_id, 2, {"b": 2})
+        assert db.query_total_sequences("p1", "batch") == 2
+
+    def test_query_total_sequences_nonexistent(self, db):
+        assert db.query_total_sequences("nope", "nope") == 0
+
 
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