Add OAI-PMH subscribe button

[eilmiv]

Summary of changes

• Added OAI-PMH subscribe button and modal with minimal explanation and OAI-PMH link

Motivation and context

Closes #1316

Screenshots

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.

[Copilot]

Pull request overview

Adds an OAI-PMH discovery/copy flow to the existing “Subscribe” UI, making the instance’s OAI-PMH endpoint easier to find and copy (per #1316).

Changes:

• Adds a new “Harvest using OAI-PMH” entry to the Subscribe dropdown for Materials.
• Introduces a Bootstrap modal that displays the OAI-PMH endpoint with a clipboard-copy button and brief guidance.
• Adds English locale strings for the new UI label and modal explanation text.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
config/locales/en.yml	Adds new i18n strings for the OAI-PMH subscribe entry and modal instructions.
app/views/subscriptions/_subscribe_to_oai_pmh_modal.html.erb	New modal partial that displays the OAI-PMH endpoint and copy-to-clipboard button.
app/views/subscriptions/_subscribe_button.html.erb	Adds the new OAI-PMH option to the Subscribe dropdown and renders the new modal for Materials.

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