Revert "fix: switch embedding dimension from 1536 to 2560 for qwen-embedding-4b"

This reverts commit e09ea3a145.

memento-note/tests/ai-provider.spec.ts

@@ -47,7 +47,7 @@ test.describe('AI Provider Configuration Tests', () => {
 
     // Verify embeddings provider is working
     expect(result.embeddingsProvider).toBe('openai');
-    expect(result.embeddingLength).toBe(2560); // OpenAI text-embedding-3-small
+    expect(result.embeddingLength).toBe(1536); // OpenAI text-embedding-3-small
     expect(result.details?.provider).toBe('OpenAI');
   });