fix: exclude ignored notation from chord diagrams - #77

Fixes #70.

Summary

• Exclude bar/repeat markers, asterisks, and N.C. from the chord names used
  to render diagrams.
• Preserve real chord names, including slash chords such as D/F#, and the
  existing annotation filtering.
• Add regression coverage for every notation form already treated as ignored
  when chord lines are parsed.

Why

ChordSheet#unique_chords feeds both the guitar and ukulele diagram renderers.
It removed bracketed annotations, but it still returned other permitted
chord-sheet notation as chord names. A bar line such as | therefore reached
the renderer and produced a missing-chord placeholder.

The updated sanitization mirrors the ignored notation used when classifying
chord lines, so D |: D/F# :| G* N.C. now yields only D, D/F#, and G.

Testing

• Full RSpec suite: 170 examples, 0 failures.
• Related model specs: 37 examples, 0 failures.
• RuboCop on the changed files: no offenses.
• Full-project RuboCop: 72 files, no offenses.
• Brakeman, Bundler Audit, Rails Best Practices, and Slim-Lint: passed.
• Cypress guitar and ukulele chord-diagram scenarios: passed on every run.

The default Rake pipeline reached Cypress after all preceding checks passed.
The full Cypress run completed 48 of 51 cases; its three failures were unrelated
textarea/sorting timing cases. Focused reruns changed the failure set, and the
exact pre-change base also reproduced existing sorting and version-history
timing failures.