Renaming "Aggregate Subject" Label to "Mean Across Subjects"

[h5hoang]

Issue

Closes #1297

Description

Renamed the "Aggregate Subject" label to "Mean across subjects" in the ratio calculations UI.

Updated the help dropdown text to explain matching mechanics (how test and reference rows are joined by grouping variables, and how the setting controls USUBJID inclusion).

Updated the tooltip and vignette documentation to match.

Change is a display-only change as the internal AggregateSubject YAML key and column name remain unchanged for backward compatibility.

Definition of Done

• Label and tooltip updated in ratio_calculations_table.R
• Column header in the ratio table updated
• Help dropdown text updated with matching explanation
• Settings YAML key remains AggregateSubject for backward compatibility (display-only change)
• NEWS entry added

How to test

1. load with devtools::load_all(); run_app()
2. upload data & navigate to NCA tab > Parameter Selection > Ratio Calculations
3. Add a ratio row
4. Hover over the —/x̄/x̄? dropdown → tooltip should say "Mean across subjects: — = no, x̄ = yes, x̄? = if-needed"

5. Click the **?** help icon → should show "Mean across subjects" with matching explanation

Contributor checklist

• Code passes lintr checks
• Code passes all unit tests
• New logic covered by unit tests
• New logic is documented
• App or package changes are reflected in NEWS
• Package version is incremented
• R script works with the new implementation (if applicable)
• Settings upload works with the new implementation (if applicable)
• If any .scss change was done, run data-raw/compile_css.R
• If a package dependency was added/changed, run data-raw/test_suggests_hidden.R

Notes to reviewer

No logic changes to the code, just purely UI label, tooltip, help text, and vignette documentation. The internal column name AggregateSubject in data frames, YAML settings, and R functions are untouched for backward compatibility.

[Gero1999]

Perfect implementation, now is perfectly clear. Good job! 😉

[KOBANAK]

I confirm it works as expected.