feat: Auto-name clusters and allow renaming them

[Harsh16gupta]

Instead of showing "Cluster 1", "Cluster 2", etc., clusters now get real names based on what's actually inside them.

How it works:

• Picks the most descriptive keywords from each cluster using TF-IDF (looks at unigrams, bigrams, and trigrams)
• Maps them to clean category names like "Travel", "Recipes", "Investment" when there's a clear match
• Falls back to the top-scoring phrase when no category fits
• Handles name collisions — if two clusters both want "Recipes", they get disambiguated

Renaming:

• Click the pencil icon next to any cluster name to rename it
• Press Enter to save, Escape to cancel
• Works in both light and dark themes

Screenshot: