Improve search suggestions and overlay scrollbar

[lycorismmoonlights]

What changed

• upgrades search suggestions to a multi-provider, canonicalized feature-ranker flow
• separates visible recent items from longer-lived search history signals
• adds suggestion debug metadata and supporting project documentation
• replaces the wide Windows Chrome root scrollbar with an auto-hiding translucent overlay while preserving native behavior on macOS and mobile

Why

The start page needed more relevant Chrome-style omnibox suggestions and a desktop scrollbar that visually matches the macOS overlay treatment without reserving a white gutter on Windows.

Validation

• node --check script.js
• node --check api/_lib/search-suggest.js
• node --check api/suggest.js
• git diff --check
• Playwright simulation for Windows Chrome overlay visibility, drag behavior, auto-hide, and zero native gutter
• separate macOS browser session confirming the custom scrollbar stays disabled