Dropdown to adjust page size. Fixes #1195

[fbacall]

Summary of changes

• Adds a page size control to the UI with some hard-coded options (10, 20, 50, 100).

Motivation and context

#1195

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

This PR adds a dropdown control to adjust the number of search results displayed per page, addressing issue #1195. Users can now select from predefined page sizes (10, 20, 50, 100) instead of being limited to the default of 10 results per page.

• Adds page size selection dropdown to the search results interface
• Introduces constants for default page size and available options
• Implements JavaScript function to handle page size changes via URL parameter updates

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
config/locales/en.yml	Adds translation key for the "Results per page" label
app/views/search/common/_search_panel.html.erb	Adds dropdown UI control for page size selection with inline form styling
app/helpers/application_helper.rb	Implements helper method to generate page size options with current selection
app/controllers/concerns/searchable_index.rb	Defines page size constants and updates default page size reference
app/assets/stylesheets/application.scss	Adds styling for the page size dropdown with custom border and transparent background
app/assets/javascripts/application.js	Implements function to update URL with selected page size and trigger page reload

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