Update rates script and add weekly icons refresh workflow

[mdrlzy]

📌 Description

• rewrote the update script for both iOS and Android
• added flags for platform selection and icons download
• updated the current rates workflow
• added a weekly workflow for rates and icons refresh

[kirillt]

@mdrlzy could you also replicate this pattern from iOS repo?

https://github.com/ARK-Builders/Rate-iOS#for-developers

The idea is to keep no rates/icons in the main branch, because that would be a confusing and potentially stale copy of live rates/icons. But we need rates/icons for local development, so they should be downloaded by developers manually right after repo cloning. And .gitignore should list the files stored in the data branch to avoid committing stale copy into main.

And the data branch must contain only data, no code.

[kirillt]

Renamings:

• data workflow to .github/workflows/fetch-data.yml
• updater script to scripts/update_crypto_data.py

Functional changes:

• Enforce CRYPTO_ASSET_LIMIT = 200
• Generate rates and crypto icons atomically in one workflow
• Retry icon fetches, don't skip and don't move beyond CRYPTO_ASSET_LIMIT
• Fail if CRYPTO_ASSET_LIMIT crypto rate/icon pairs cannot be built
• Checkout main for scripts before updating exchange-rates
• Restore exchange-rates assets before build and release

Extra validations:

• Validate downloaded crypto icons are PNG files
• Support coin_ drawable names for digit-prefixed tickers
• Normalize COIN_ icon codes back to ticker codes
• Fix CURR_TRY icon-code normalization
• Add Python bytecode cache ignores to .gitignore