Override headers

[sebpretzer]

Fixes: #212 and #226

1. Updates the settings to allow users to pass in custom headers
2. Adds documentation to show users how to do so
3. Adds a warning message when it occurs in code to point to the documentation

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/pyticktick
settings.py					393-402
Project Total

This report was generated by python-coverage-comment-action

[sebpretzer]

self-review complete

[Copilot]

Pull request overview

This PR adds support for overriding V2 API request headers to address issues where TickTick returns 429 errors due to outdated headers. The changes enable users to customize the User-Agent and X-Device headers when the default values become invalid.

• Introduces configurable v2_user_agent and v2_x_device settings with sensible defaults
• Adds warning message when 429 errors occur to guide users to header override documentation
• Provides comprehensive documentation explaining how to find and set valid header values

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

File	Description
src/pyticktick/settings.py	Adds V2XDevice model and configurable header fields; removes hardcoded header values; adds warning for 429 errors
tests/unit/test_settings.py	Adds comprehensive test coverage for new header configuration functionality
docs/guides/settings/overriding_outdated_headers.md	New documentation explaining header override feature with examples
mkdocs.yaml	Adds navigation entry for new documentation page

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