docs(sdk): clarify OAuth vs bearer token auth for frontend vs backend

[rickyrombo]

Summary

• Removes the misleading HTML+JS initialization example that showed bearerToken in browser code (directly contradicting the warning below it)
• Splits the "Initialize the SDK" section into clearly labeled backend vs. frontend examples
• Adds a new "Log In with Audius (OAuth)" section with a concrete code example for the PKCE flow
• Updates the security warning to be more explicit and link to the OAuth section
• Clarifies in the "Direct API Access" section that bearer token curl/gRPC examples are backend-only

Test plan

• Verify Node.js example still shows apiKey + bearerToken
• Verify HTML+JS example shows apiKey only (no bearer token)
• Verify OAuth section code example is syntactically correct
• Verify all links resolve correctly in docs site

🤖 Generated with Claude Code

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