Infer annotation aesthetics from the active ggplot2 theme (#120)

[ishaan-arora-1]

Fixes #120.

Reference lines like the vertical zero-line in mcmc_intervals() and the rhat=1 line in mcmc_rhat() had hardcoded colors ("gray90", "gray") that didn't adapt when users switched ggplot2 themes. As @tjmahr pointed out in the issue, this makes them invisible or look wrong on themes like theme_grey() or dark themes.

Added an internal annotation_style() helper that reads the active theme's gridline color and linewidth to derive sensible reference line aesthetics. When gridlines are blank (bayesplot's default theme), it falls back to the previous gray90/0.5 defaults so existing plots look exactly the same.

Updated the four internal call sites in mcmc-intervals.R (3x vline_0) and mcmc-diagnostics.R (1x vline_at) to use the new helper.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 43.47826% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.35%. Comparing base (c817061) to head (a0fd434).
⚠️ Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
R/helpers-gg.R	26.66%	11 Missing ⚠️
R/mcmc-diagnostics.R	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
- Coverage   98.63%   98.35%   -0.29%     
==========================================
  Files          35       35              
  Lines        5860     5883      +23     
==========================================
+ Hits         5780     5786       +6     
- Misses         80       97      +17     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.