feat: make supported chains list dynamic via getSupportedChains API

[stephancill]

What changed? Why?
The supported chains list was hardcoded in two places with inconsistencies between them. This PR:

• Creates a shared docs/snippets/supported-chains.mdx snippet used by both the Base Account overview and mini-apps compatibility pages
• Adds scripts/update-supported-chains.sh that fetches the SCW getSupportedChains API and injects any chains tagged simple-chain into the snippet
• Core chains remain hardcoded; only basic chains (tagged simple-chain) are dynamic
• Basic chains section includes a note that they are only available on Base Account Web

Notes to reviewers
No simple-chain tags exist in prod yet, so the dynamic section is currently empty. When basic chains are tagged in scw-manager, running the script will populate them. Tested against the dev API (wallet-api-dev.cbhq.net) where Monad, Unichain, and ZKsync are tagged.
The script supports an API_URL env var override for testing: API_URL=https://wallet-api-dev.cbhq.net/rpc/v3/scw/getSupportedChains bash scripts/update-supported-chains.sh

How has it been tested?
Ran mintlify dev locally and verified rendering with both prod API (no basic chains) and dev API (Monad, Unichain, ZKsync shown).

[cb-heimdall]

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