feat(web): warn before leaving an instrument whose responses are unsaved

[joshunrau]

Closes #717.

Instrument state is not saved until it is submitted. A stray sidebar click, a refresh or a closed tab discarded the clinician's answers without asking. Now, from "Begin" until the summary, leaving the page asks for confirmation first (Yes / No dialog in the app, the browser's native prompt on refresh or close). This covers both scalar and series instruments. Before, the warning appeared only while a file upload was pending.

What changed

• One blocker, owned by InstrumentRendererContainer. Both renderers pass it the injected NavigationBlocker, and it is active while the content step shows (index === 1). SeriesInstrumentRenderer takes the prop now. The upload-only blocker in FileInstrumentContent is gone. TanStack runs every registered blocker in turn, so keeping it would have shown a second dialog during uploads. Uploads are still covered, because the renderer stays on the content step until onSubmit resolves.
• NavigationBlocker (web) holds beforeunload only while active. It passed enableBeforeUnload: true, so refresh prompted for as long as the blocker was mounted. That was harmless while it lived only inside the file upload screen, but it would have prompted on the overview and summary once mounted for the whole instrument.
• Ending a session bypasses the blocker (ignoreBlocker in Sidebar, Navbar, and the render page's no-session redirect). Sidebar's End Session already confirms, and endSession() runs before the navigation. Blocking would leave the clinician on a form with no session, where submit fails on currentSession!. The e2e test reproduced exactly that before the fix.
• packages/react-core/AGENTS.md updated: it said only the scalar path takes NavigationBlocker.

Verification

• Unit (react-core): ScalarInstrumentRenderer.test.tsx checks the blocker is inactive on the overview, active once begun, still active while files upload, and inactive at the summary. SeriesInstrumentRenderer.test.tsx checks it stays active between items and is released at completion.
• Unit (web): src/__tests__/navigation-blocker.test.tsx runs against a real router. It covers beforeunload while active and inactive, "No" keeping the location, and "Yes" completing the navigation.
• E2E: testing/src/specs/instrument-navigation-block.spec.ts. Declining keeps the clinician on the instrument with answers intact. Confirming leaves. No warning after submission. End Session skips the warning and ends the session.
• I made each new test fail on purpose: block on summary, never block, enableBeforeUnload: true, and dropping ignoreBlocker.
• pnpm lint, pnpm test (1634 passed) and pnpm test:e2e (276 passed) are green locally.

Not covered

• Gateway (remote assignments) has no router and passes no NavigationBlocker, so patients get no warning on refresh or close. That's out of scope here (Area: Web) and would need its own beforeunload-only blocker.
• The mobile Navbar's End Session still has no confirmation of its own, so it now ends a started instrument without any prompt (same as before this PR).
• The leave dialog appears only for navigations inside the app or page unloads. Interactive instruments run in an iframe, and links inside the iframe are not intercepted.

Co-Authored-By: Claude Opus 5.5