fix(internet-identity): hardcode II canister ID, add fetch binding

[marc0olo]

Summary

• Replace wrong fallback canister ID (be2us-64aaa...) with the well-known II canister ID (rdmx6-jaaaa-aaaaa-aaadq-cai) — same on mainnet and local replicas, no dynamic lookup needed
• Add fetch: window.fetch.bind(window) to HttpAgent.create() to prevent "Illegal invocation" errors in bundled builds (Vite, webpack)
• Add new pitfall documenting the fetch binding requirement

[marc0olo]

@sea-snake mentioned that Add fetch: window.fetch.bind(window) to HttpAgent.create() to prevent "Illegal invocation" errors in bundled builds (Vite, webpack) should not be required and that this might be an issue in the JS SDK (cc @raymondk @nikosxenakis)

[nikosxenakis]

@sea-snake mentioned that Add fetch: window.fetch.bind(window) to HttpAgent.create() to prevent "Illegal invocation" errors in bundled builds (Vite, webpack) should not be required and that this might be an issue in the JS SDK (cc @raymondk @nikosxenakis)

This PR moves the binding of the window inside the agent constructor so consumers don't need to explicitly initialise it dfinity/icp-js-core#1316

[marc0olo]

Superseded by #101 which addresses both changes from this PR:

• Hardcoded II canister ID — applied in fix(internet-identity): rewrite skill to fix agent hallucinations #101
• fetch: window.fetch.bind(window) — not needed, fixed upstream in icp-js-core#1316