Fix best-metric highlighting for LL/AIC/BIC and add "always" to update_check options

[Copilot]

Three bugs from the ab132e5 review: LL/AIC/BIC "best" detection used closest-to-0 instead of minimum, and "always" was a supported update_check value that couldn't be persisted through the UI.

Changes

• R/PM_compare.R + R/PMutilities.R — Replace single-target heuristic with three-way logic:

  • LL$|AIC$|BIC$ → min(x) (smallest is best)
  • Sl|R2 → closest to 1
  • everything else (bias, imprecision, intercepts) → closest to 0

  # Before: all non-slope/R2 defaulted to closest-to-0, mis-ranking LL/AIC/BIC
  target <- ifelse(grepl("Sl|R2", col), 1, 0)
  valid[which(abs(x[valid] - target) == min(abs(x[valid] - target)))]
  
  # After
  if (grepl("LL$|AIC$|BIC$", col, ignore.case = TRUE)) {
    valid[which(x[valid] == min(x[valid]))]
  } else if (grepl("Sl|R2", col)) {
    diffs <- abs(x[valid] - 1); valid[which(diffs == min(diffs))]
  } else {
    diffs <- abs(x[valid]); valid[which(diffs == min(diffs))]
  }

• R/PMoptions.R — Add "always" to the validation allowlist and to the Shiny UI selectInput choices, so it can round-trip through the options UI (it was already handled by pm_update_interval_days() but silently replaced with "weekly" on save).

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