docs: improve OAuth error 403 troubleshooting and test user setup

[dandaka]

Summary

Improves the API authentication documentation to help users avoid the common Error 403: access_denied issue during OAuth setup.

Problem

Many users encounter the error "Access blocked: gcalcli has not completed the Google verification process" when first setting up gcalcli. This happens when they attempt to authenticate before adding their email address as a test user in the OAuth consent screen configuration.

Changes

• Added troubleshooting section for Error 403 with clear solution steps
• Clarified OAuth consent screen configuration - explicitly list the test user setup steps
• Added note about "Testing" mode - explain that apps remain in testing mode for personal use
• Improved OAuth flow expectations - explain the "Google hasn't verified this app" warning is expected and safe

Why This Matters

This change addresses a common setup issue that blocks users from completing the initial authentication flow. By clarifying the test user requirement upfront and providing troubleshooting guidance, users can complete setup successfully on their first attempt.

Generated with Claude Code

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This pull request improves the documentation for API authentication by:

• Enhancing OAuth2 Consent Screen Setup Instructions: Provides more detailed steps for creating the OAuth2 consent screen, including selecting "External" user type, clarifying the order of information entry, and explicitly detailing the process of adding test users. It also adds a note explaining that the app will remain in "Testing" mode for personal use.
• Updating OAuth Flow Warning Description: Refines the description of the security warning users will encounter during the OAuth flow, specifying it as "Google hasn't verified this app" and guiding users to click "Continue" (potentially after "Advanced").
• Adding Troubleshooting for Error 403: Introduces a new troubleshooting section specifically addressing Error 403: access_denied when "gcalcli has not completed the Google verification process." It explains that this error occurs if the authenticating email is not added as a test user and provides a clear solution to add the email in the OAuth consent screen.

[kody-ai]

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