refactor: replace getHeadersWidth's out-of-range loop-index probe with hasFrozenColumns()

[ghiscoding]

Port bug fix from 6pac/SlickGrid PR 6pac/SlickGrid#1260 into slickgrid-universal

The quirk

getHeadersWidth()'s post-loop scrollbar attribution re-tested the loop variable after the loop ended — i === columns.length, an out-of-range index probe:

if ((this._options.frozenColumn!) > -1 && (i > this._options.frozenColumn!)) {
  this.headersWidthR += scrollbarWidth;   // i is columns.length here
}

This only happened to be equivalent to 'is a left freeze active' because setFrozenOptions clamps frozenColumn to < columns.length, making the second conjunct invariantly true whenever the first is — an accidental invariant a future refactor could silently break.

The change

State the intent directly with hasFrozenColumns() and scope the loop variable into the loop so the accidental dependency cannot recur. Zero behavior change for every reachable state (equivalence follows from the setFrozenOptions clamp).

Per the triage discussion this PR deliberately does not touch the double scrollbar addition further down in the same function (headersWidthR/L get the scrollbar added again in the band-finalization block) — that one has potential pixel-visible effects and deserves its own investigation.

Why no repro/test

Nothing observable changes — there is no symptom to pin. Width-sensitive suites (frozen-columns-and-rows, example1, auto-resize) ran 18/18 green as a regression gate.

(Part of the quirks-triage series:
6pac/SlickGrid#1255
6pac/SlickGrid#1256
6pac/SlickGrid#1257
6pac/SlickGrid#1258
6pac/SlickGrid#1259

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (15d9833) to head (116fc3b).
⚠️ Report is 15 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2675   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         200      200           
  Lines       25452    25461    +9     
  Branches     8999     9000    +1     
=======================================
+ Hits        25452    25461    +9     

Flag	Coverage Δ
angular	100.0% <ø> (ø)
universal	100.0% <100.0%> (ø)

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commit: 116fc3b