Feat: Replace Load More with Previous/Next pagination in library

Page size changes from 50 to 24. Library now shows discrete page navigation
with a "Page N of M" indicator, total image count, and URL state (?page=N)
so pages are bookmarkable and the browser Back button works. Tag filter
resets to page 1. Out-of-range page params are clamped silently.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/karma.conf.js

@@ -5,6 +5,7 @@ module.exports = function (config) {
     plugins: [
       require('karma-jasmine'),
       require('karma-chrome-launcher'),
+      require('karma-firefox-launcher'),
       require('karma-jasmine-html-reporter'),
       require('karma-coverage'),
       require('@angular-devkit/build-angular/plugins/karma'),