Add widget page support for placeholder-plain

[sestr]

The widget for a signature could only be placed on the first page of a PDF document with the placeholder-plain package. However, it should also be possible to insert the widget on any other page. I made this possible by adding the widgetPage parameter to the configuration object of the plainAddPlaceholder method.

[coveralls]

coverage: 99.319% (-0.3%) from 99.636%
when pulling 0f20522 on sestr:feature/widget-page
into 4a6a2a4 on vbuch:develop.

[vbuch]

This PR seems like something completely valid. I don't know why it was never reviewed. @sestr any interest in rebasing so we can get this merged?

The only possibility I think would be even better is to support the signature being shown on every page. Not part of this PR in any way but an interesting feature to have. It should be doable as long as there are /Widgets on every page all linking to the same /Sig.

[sestr]

No problem @vbuch, I just did a rebase.

Adding a signature to every page is indeed an interesting idea for an additional feature. This could be used, for example, to initial each page of a contract.

[coveralls]

coverage: 99.47% (-0.5%) from 100.0%
when pulling 987cc53 on sestr:feature/widget-page
into f54ca67 on vbuch:develop.

[Copilot]

Pull Request Overview

This PR adds support for specifying a target page when inserting a signature widget via plainAddPlaceholder.

• Introduces a new widgetPage parameter in the configuration and JSDoc for plainAddPlaceholder
• Updates plainAddPlaceholder to pass widgetPage through to getPageRef
• Enhances getPageRef to accept an optional pageNumber, split out page references, and validate bounds

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

File	Description
packages/placeholder-plain/src/plainAddPlaceholder.js	Added widgetPage to JSDoc, function signature defaults, and parameter propagation
packages/placeholder-plain/src/getPageRef.js	Extended getPageRef signature, reworked page reference parsing, and added bounds checks

Comments suppressed due to low confidence (4)

packages/placeholder-plain/src/getPageRef.js:10

• Use lowercase {number} in JSDoc for consistency with other annotations.

* @param {Number} [pageNumber = 0] Desired page number

packages/placeholder-plain/src/getPageRef.js:22

• [nitpick] Enhance this error message by including the total page count (e.g., Page ${pageNumber} out of ${pagesSplit.length} pages) to aid debugging.

throw new SignPdfError(`Failed to get reference of page "${pageNumber}".`, SignPdfError.TYPE_INPUT);

packages/placeholder-plain/src/getPageRef.js:20

• Consider splitting on whitespace with split(/\s+/) instead of a single space to avoid empty tokens if there are multiple spaces or line breaks.

pages.trim().split(' ').forEach((v, i) => (i % 3 === 0 ? pagesSplit.push([v]) : pagesSplit[pagesSplit.length - 1].push(v)));

packages/placeholder-plain/src/getPageRef.js:19

• [nitpick] You could simplify the chunking logic by iterating over tokens in steps of 3 (e.g., for (let i = 0; i < tokens.length; i += 3) pagesSplit.push(tokens.slice(i, i + 3));) for clearer intent.

const pagesSplit = [];