fix: resolve async embedding generation and error handling issues

[Copilot]

Addresses unresolved code review comments regarding blocking operations, error handling, and resource cleanup.

Async Embedding Generation

Clipboard monitoring was blocked by synchronous OpenAI API calls. Items are now saved immediately; embeddings generate asynchronously and update the database post-insertion:

// Save item immediately without embedding
const id = this.db.addItem(item);
item.id = id;
this.history.unshift(item);

// Generate embedding asynchronously - non-blocking
this.embeddingService.getEmbedding(text)
  .then(embedding => {
    if (embedding?.length > 0) {
      this.db.updateItemEmbedding(id, embedding);
    }
  })
  .catch(error => log.error("Embedding failed:", error));

Error Handling & UX

• Semantic search: Shows explicit error when API key missing instead of silent failure
• Settings timeout: Added useEffect cleanup to prevent state updates on unmounted component
• hasMore state: Resets when new items arrive or search clears

Logging & Database

• Replaced console.log with electron-log/renderer for production consistency
• Added updateItemEmbedding() method to DatabaseManager
• Fixed semantic search to use Buffer format instead of JSON for sqlite-vec compatibility

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