test: lift frontend coverage to 80%+ and enforce threshold

[Uday9909]

Summary

Raises the webapp's vitest coverage from 59.7% → 95.9% statements and wires an enforced 80% coverage threshold into vite.config.js so CI fails on regression.

Changes

• Hooks (useSession, useStreamingOutput): full lifecycle tests (create/end/error/expiry/streaming, 1000-line cap, unmount cleanup).
• Pages: Login (render + navigate), Demo (mock mode, live mode, 503/429/404/409 errors, reset, getMockResponse branches).
• Debug panels (Functions, Stack, Breakpoint, BreakPoints, MemoryMap, Threads): loading / error / no-session / fetch-success / fetch-error states.
• api.js: request interceptor (session_id injection) + response interceptor (404 → session-expiry callback) via a capture adapter.
• src/setupTests.js: stub Element.prototype.scrollIntoView (jsdom doesn't implement it; Demo's log panel calls it in an effect).
• vite.config.js: add coverage.thresholds (statements/branches/lines 80, functions 75) so the existing CI coverage step gates on the target.

Results

• 115 tests passing (was 57)
• Statements 95.85% · Branch 91.6% · Lines 95.85% · Functions 77%
• ESLint: 0 errors · vite build: green

Test plan

• npx vitest run --coverage — 115 pass, thresholds enforced
• npx eslint src/ — 0 errors
• npm run build — green