Slider style-label for= handling: consolidate 21 duplicated sites (JS init pass or shared PHP helper)

[vivi-the-going-merry]

Follow-up from PR #3468's review (two non-blocking notes from the same round):

1. Every one of the 21 data-slider-label-for sites in classes/views/styles/ duplicates the decision to print a conditional for="..." as a per-site PHP conditional keyed off FrmSliderStyleComponent::is_value_measured(), when slider-component.js's initListeners() already resolves valueInput.disabled for every slider at page load via the same underlying is_measured_unit() logic (slider.php:132's disabled() call). A single JS init-time pass — find the label via the existing [data-slider-label-for="..."] marker and call updateLabelFocusTarget(valueInput, !valueInput.disabled) once per slider — would let every template keep a plain static for and never need the conditional at all.

2. Separately, each of the 21 sites hand-types the {id}-value string twice (once in data-slider-label-for, once in the conditional for), plus a third time in the FrmSliderStyleComponent constructor's 'id' option. A small static helper on FrmSliderStyleComponent (e.g. label_for_attrs( $id, $value ) printing both attributes from one $id) would cut the two adjacent literals in the <label> down to one, at all 21 sites. If going this route, note phpcs.xml:76's WordPress.Security.EscapeOutput.OutputNotEscaped sniff will need the helper allowlisted (or self-escaping) since it'd be echoed directly, same as FrmAppHelper::array_to_html_params() already handles elsewhere in this codebase.

Doing (1) first would make (2) unnecessary — the JS-side pass removes the PHP conditional entirely, so there'd be nothing left needing a helper. Worth deciding as one piece of work rather than building both.

Neither note blocks #3468 — Franky approved that PR; these are structural cleanup for a future pass.

[vivi-the-going-merry]

Blocked on #3468 merging first — that's still open (state: OPEN, unmerged), and it's itself stacked on #3449's branch (also open, unmerged, targeting master), not on master directly. The 21 data-slider-label-for sites this issue describes don't exist on master yet, so there's nothing to consolidate against yet.

Re-surface this once #3468 (and #3449 underneath it) land on master.