Decouple projection and KMeans in embed_explore app #25

The precalculated app was refactored in #24 to separate dim reduction from KMeans so users can project once and run multiple k values without re-projecting. The embed_explore app still couples them in a single "Run Clustering" action.

We'd like to mirror the precalculated app's split:

• Replace render_clustering_section() in apps/embed_explore/components/sidebar.py with separate render_projection_section() and render_kmeans_section()
• Use existing helpers from Separate dim reduction and KMeans into independent operations #24: ClusteringService.run_dim_reduction_safe() and run_kmeans_only_safe()
• Replace hardcoded color_col = 'cluster' in shared/components/visualization.py:127 with the "Color by" dropdown pattern. Color by different KMeans cluster result
• Preserve KMeans columns on re-projection with PCA, TSNE, UMAP