108 migrate omeka consumers to a new provider

[Nealium104]

This PR migrates ExploreUK consumers off OmekaShim.php and onto ContentProvider.php and Config.php. With OmekaShim.php deleted, the application no longer has any omeka-specific code paths. Container configs and infra still reference omeka by name; those are separate cleanups tracked under #113.

Changes

• Consumer migration: Replaces all OmekaShim.php calls in ExploreUK.php, Oai.php, View.php, catalog.php, and the view templates (front-page, header, global-footer, search, sponsors, universal-footer) with direct reads against Config.php / ContentProvider.php. Deletes OmekaShim.php.
• Asset relocation: Collapses app/themes/omeukaprologue/ and app/themes/assets/ into a single app/assets/ tree. Updates docker volume mounts (./app/themes, ./app/assets) and the minify-css script paths in app/package.json and .gitignore.
• JS directory consolidation: Merges javascripts/ and assets/js/ into a single app/assets/js/. Updates the PHP references that pointed at javascripts/.
• Dead omeka theme files removed: config.ini, theme.ini, theme.jpg, a duplicate styles.css, and two unused CSS image assets. There is still more to be removed, but final cleanup takes place as part of a separate subissue
• Tests: ConfigTest and ContentProviderTest updated to match the new shape. Full suite passes (18 tests, 38 assertions).

Notes for review

• The relocation commit (9bc1fe8) is mostly rename noise. The substantive diff lives in 56a765e (path fixups) and 13e9f9b (consumer migration).
• SOLR_URL is committed to .env.example with example values as documentation, but production values in .env.dev is not included in this PR. Whether .env.dev should include the production solr url is an open question. It is a manual step to add the production solr url to a given setup.

fixes #108