fix(website): show loading feedback on Scan & Visualize - #1757

Problem

On the Client Indexer -> GitHub / GitLab tab, clicking Scan & Visualize gave no feedback. The button only called navigate() to the /explore route, which does the actual fetch + index work in a separate page (Explore.tsx). Nothing in LocalUploader.tsx tracked an in-flight state for that click, so the button stayed enabled with unchanged text for the whole gap between click and the next page rendering. Rapid clicks could queue multiple navigations (confirmed: two clicks grew browser history by 2 entries instead of 1).

Fix

Wrap the navigate() call in React's useTransition, and drive the button's disabled state and a "Scanning repository..." spinner off its pending flag:

- navigate(getExploreRoute(ref));
+ startScan(() => navigate(getExploreRoute(ref)));

useTransition was chosen over a plain isScanning boolean because navigate() is synchronous (no promise to await/finally on), and some paths never reach a success/error callback that could reset a manual flag — e.g. a route segment that causes Explore to skip auto-indexing, or the user pressing Back while the lazy Explore chunk is still loading. The transition's pending state always resolves itself once navigation settles, so the button can't get stuck disabled.

Also added a re-entry guard (if (isScanning) return;) at the top of the handler as defense in depth alongside the disabled button.

This only touches website/src/components/LocalUploader.tsx (16 lines added / 4 changed). No backend/API or fetch/index logic was touched, and the existing success/error handling in Explore.tsx (loader, error screen with Retry) is unchanged.

Testing

• Ran the dev server in headless Chrome and compared before/after on: home page widget and /explore page, double-click, invalid/empty input, a route segment that skips auto-indexing, pressing Back mid-load, and retry after a simulated fetch failure. Button now disables immediately and shows the spinner in every case, and re-enables correctly after both success and failure paths.
• tsc --noEmit: same pre-existing errors elsewhere in the website package, none in this file.
• eslint on the changed file: same pre-existing errors (unrelated any usages etc.), no new ones.
• vite build: passes.
• No test framework exists in website/ (no vitest/testing-library configured), so no automated test was added for this UI-only change.

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