Add missing Pokémon sprites

[anhthang]

This PR adds tooling and assets to help complete missing sprites:

• Add a check_missing_sprites.py script that reads Pokémon and forms CSVs, determines generation via version groups, and reports/export CSVs of missing front/back (normal and shiny) sprites.
• Add a smogon_download.py script to download missing sprites from the Smogon attachment URLs, then use renameSmogon.sh (updated to work from the scripts/downloads folder and move files instead of copy) to place them into the correct sprite directories.
• Manually add sprites for special Imgur URLs and forms with pokemon_id ≥ 10000 that are not covered by the automated scripts.

[Naramsim]

Hi, thanks for the contribution! I currently don't have much time to review it, please give me some days.

[anhthang]

No worries at all, this PR turned out pretty big and I know it will take time to go through everything. Please take your time with the review.

I’ve tried to be careful with the Smogon links and file placements, so hopefully everything looks good there, but I’m happy to adjust anything you spot.

[Naramsim]

@anhthang The PR is too big to be reviewed, at least by me. Could you come up with a separate smaller one if possible (related to one script only maybe), so that it's going to be easier to review?

[anhthang]

@Naramsim This PR only depends on the download script and an update to the existing rename script to better handle forms/varieties. The check-missing script can be ignored for review purposes — it’s only used to identify missing assets and doesn’t affect the main workflow.

If splitting the PR would make it easier to review, which approach would you prefer? I could split the updates per generation sheet. However, Gen 9 might still be quite large since it includes many updates.

Another, though it may not be ideal, is reviewing by commit, since the changes are already logically separated across commits.