Add a search bar to the event spread view

[moveson]

Spectators and crews — typically on phones — often need to locate a single entrant on the full spread of a large event. Scanning a long table is slow, and the browser's find-in-page feature is not discoverable on mobile. A search bar at the top of the spread would solve this directly.

The backend already supports it

EventSpreadDisplay#filtered_ranked_efforts (app/presenters/event_spread_display.rb) already applies .search(search_text), backed by the existing pg_search scopes (search_names_and_locations, search_bib). The page simply never grew a search input, so this is a view-layer feature.

Tasks

• Add a search input to the spread view (reuse the house search-input pattern from other filtered index pages), submitting via GET so results are shareable/bookmarkable and display_style/sort params are preserved
• Add search to EventSpreadDisplay#cacheable_params_digest — the digest currently covers only display_style, filter, and sort, so a searched result set would be cached under (or served from) the unsearched cache entry. This is technically a latent bug today for hand-built URLs and becomes load-bearing once search is in the UI
• Show a clear-search affordance and an empty-state message when no entrants match
• Mobile-first placement/sizing — the primary user is a crew member on a phone

Notes

• Each distinct search term becomes its own fragment-cache entry once the digest includes it; acceptable at current traffic, but worth keeping the digest normalization (e.g. strip/downcase) in mind
• Related but independent: highlighting followed entrants for signed-in users is tracked separately

[moveson]

@yanatan16 for visibility