[WIP] Fix split string suggestion labels to improve translator flexibility

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>i18n: split-string suggestion labels reduce translator flexibility</issue_title>
<issue_description>## Background
PRs #369 and #378 fixed user-visible bugs where the suggestion form / sidebar rendered literal `{{changeFrom}}` placeholders. The fix in both cases was to split the original sentence

Suggested change from "{{changeFrom}}" to "{{changeTo}}"

into three pieces — a `_from_label` key ("Suggested change from"), the value span, a `_to_label` key ("to"), and the second value span — because html10n's `data-l10n-args` substitution wasn't reliably replacing `{{…}}` placeholders.

Why this is suboptimal

Splitting the sentence costs translator flexibility. Languages with verb-final word order, RTL flow, or different "from … to …" idioms can no longer reorder the values relative to the surrounding text — the DOM order is fixed.

Root cause to investigate

`vendors/html10n.ts` line ~631 does `JSON.parse(node.getAttribute('data-l10n-args'))` with no try/catch. If the selected text contains a quote or backslash, jquery.tmpl's `${changeFrom}` substitution produces invalid JSON and the parse throws — html10n then renders the unsubstituted `{{changeFrom}}` (or fails silently depending on the call site).

Suggested fix

1. Wrap the `JSON.parse` in a try/catch with a console.warn fallback so a single broken element doesn't break the whole element list.
2. Build `data-l10n-args` from JS rather than from the template (e.g. `$el.attr('data-l10n-args', JSON.stringify({changeFrom: selectedText}))` after tmpl render) so the values are always valid JSON-encoded strings.
3. Once that's reliable, reunify the split keys back into the original single-string translations and delete the `_from_label` / `_to_label` workaround keys.

Scope

• `templates/comments.html` — display-suggestion + new-comment-form blocks
• `static/js/index.js`, `static/js/newComment.js`, `static/js/index.js#displayNewCommentForm` — set data-l10n-args at runtime
• `locales/en.json` — restore single-string keys, deprecate split keys
• Coordinate with translatewiki sweep so existing translations of the original `suggested_change_from` / `suggest_change_from` keys are reused</issue_description>

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