Add Pokémon dimensionality reduction explorer

[jbkunst]

What

• adds a Pokémon-themed Shiny app focused on one large Highcharter scatter
• lets users switch between PCA, t-SNE, and UMAP
• exposes t-SNE perplexity/iterations and UMAP neighbors/minimum distance
• uses Pokémon sprites as chart markers and a rich HTML tooltip with official artwork, types, generation, size, battle stats, capture rate, happiness, growth, habitat, hatch counter, and special status
• includes Pokémon-inspired dashboard styling based on the spirit of the old flexdashboard example

Data preparation

• prepare_data.R downloads maintained PokeAPI CSV tables and builds the joined Pokémon dataset
• includes battle stats, height, weight, base experience, capture rate, base happiness, hatch counter, gender rate, baby/legendary/mythical flags, growth rate, body color, body shape, habitat, types, and egg groups
• no arbitrary rarity score is invented: capture rate plus legendary/mythical flags remain explicit source variables

Mixed-data preprocessing

• continuous features are median-imputed and standardized
• binary flags stay as 0/1; rare flags are deliberately not z-scored
• PokeAPI gender_rate = -1 is handled as a separate genderless flag plus a bounded female-ratio feature
• nominal variables are one-hot encoded as 0/1 indicators
• semantic blocks are normalized by 1 / sqrt(number of columns) so a dummy-heavy block does not dominate Euclidean distance merely because it creates more columns
• block weights are explicit and default to 1 so sensitivity can be reviewed later
• PCA centers the final matrix but does not rescale it again; t-SNE and UMAP consume the same prepared Euclidean feature space

Runtime

• marked Runtime: server, consistent with the existing dimensionality-reduction app and its Rtsne/uwot dependencies
• embeddings are intentionally computed at runtime so parameters remain interactive

Validation

• reviewed statically against the current repository conventions
• not executed locally in this environment because an R runtime is not available here
• no generated gallery/docs files are included
• no empty commits