fix: recover from stale UI bundle after upgrade instead of showing 50X

[rstrzelecki-inne-projekty]

Proposed Changes

Problem. After an upgrade, every browser tab that was opened before it still runs the previous UI bundle. When the user navigates to a route whose page chunk has not been loaded yet, the bundle requests the old hashed file (e.g. /static/js/6487.02e56745.chunk.js), which no longer exists. gin's StaticFS falls through to NoRoute, which answers with index.html and HTTP 200, so the browser tries to parse HTML as JavaScript, the lazy import() rejects and the route errorElement renders the "HTTP 50X" error page. The API is fine the whole time; only a hard refresh fixes the tab, which is confusing for users right after every deploy.

Changes.

• internal/router/ui.go: a request for a missing file under /static/ (honouring baseURLPath) now answers 404 instead of index.html — in both the embedded-build handler and the ANSWER_STATIC_PATH handler. Unit test added (ui_test.go).
• ui/src/router/index.tsx: when the lazy page import fails, the tab reloads once (a sessionStorage flag prevents a loop; it is cleared after the next successful import), so the new index.html and chunks are picked up automatically instead of showing the 50X page.

How to reproduce: open the site, deploy a new build (chunk hashes change), then navigate in the old tab to a page you have not visited yet → "HTTP 50X". With this change the tab reloads itself and shows the page.

Tested on a production instance (2.0.2 + this patch): simulated by rewriting a chunk URL to a non-existent hash — the tab reloaded once and rendered the page; SPA routes (/questions, /users/...) still get index.html, existing assets still return 200.

Checklist

• Go code builds and go test ./internal/router/ passes
• pnpm lint / tsc --noEmit clean for the UI change

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https://claude.ai/code/session_0181uZ2px83HmPo3HKSEzWQR