Story Share portal: live search suggestions (autocomplete)

Context

The Story Share portal (/story_shares) navbar has a keyword search that submits to a full results page (?query=…). The live WordPress site (stories.awbw.org) additionally shows an AJAX live-suggestion dropdown (JetSearch) as you type, surfacing matching story titles before you submit.

What's needed

• As-you-type suggestions below the portal search box: matching story titles (respecting the public/publicly_visible policy scope), each linking to the story.
• Debounced requests to a lightweight JSON/Turbo-Stream endpoint on StorySharesController (or a small suggestions action), reusing Story.search_by_params/SearchCop.
• Keyboard navigation (up/down/enter/escape) and outside-click dismiss; accessible (ARIA combobox/listbox).
• Prefer Turbo (frame/stream) over bespoke JS where possible; a small Stimulus controller for debounce + keyboard handling is acceptable.
• Must work on mobile (the portal search appears in the mobile menu too).

Notes

Plain submit-to-results search already ships in the portal-parity work; this issue is the progressive-enhancement layer on top.

Start at the /story_shares navbar search and StorySharesController, then inspect the existing Story.search_by_params/SearchCop flow and the mobile-menu search. Add the progressive live-suggestion behavior with the stated visibility scope, keyboard and ARIA requirements, and verify that suggestions link to stories and work in both desktop and mobile search.