Add singular/plural support for QA notification messages

[oscar-escire]

References

• Fixes Incorrect plural in translation for single pending suggestion #4202
• Malware-Free PR of fix(i18n): add singular/plural support for QA notification messages #5051

Description

This PR fixes the incorrect pluralization in QA notification messages. Previously, when exactly 1 suggestion was present, the message incorrectly displayed "There are 1 pending suggestions" instead of the grammatically correct singular form.

Instructions for Reviewers

1. Log in as an administrator
2. Navigate to My DSpace page
3. Verify QA notification messages:
   • Sources with 1 suggestion should display: "There is 1 pending suggestion related to your account"
   • Sources with 2+ suggestions should display: "There are X pending suggestions related to your account"

List of changes in this PR:

• Added separate translation keys with .one and .other suffixes for singular/plural forms in en.json5 and de.json5
• Updated qa-event-notification.component.html to conditionally select the correct translation key based on totalEvents count
• Updated my-dspace-qa-events-notifications.component.html with the same pluralization logic

Note: Testing requires QA events configured in the backend.

Checklist

• My PR is created against the main branch of code (unless it is a backport or is fixing an issue specific to an older branch).
• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• My PR passes ESLint validation using npm run lint
• My PR doesn't introduce circular dependencies (verified via npm run check-circ-deps)
• My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• My PR aligns with Accessibility guidelines if it makes changes to the user interface.
• My PR uses i18n (internationalization) keys instead of hardcoded English text, to allow for translations.
• My PR includes details on how to test it. I've provided clear instructions to reviewers on how to successfully test this fix or feature.
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• If my PR fixes an issue ticket, I've linked them together.

[lgeggleston]

@MMilosz since you assigned yourself for review on the original PR for this, would you still be open to being a reviewer on this? Thanks!