feat(app): inspector polish — loading states, responsive, keyboard nav

[sepehr-safari]

Summary

Polishes the Inspector UI with loading states, responsive layout, and keyboard navigation.

Closes #29

Changes

Loading states

• Spinner with "Parsing trace…" indicator during analysis
• Analyze button shows "Analyzing…" and disables while parsing
• Uses setTimeout(0) to let the loading state render before heavy synchronous parsing

Responsive layout (mobile-friendly)

• Flex-wrap for sample scenario buttons on small screens
• Smaller text sizes on mobile (text-xs → text-sm/sm:text-base breakpoints)
• Adaptive grid: grid-cols-2 sm:grid-cols-4 for stat cards
• Shorter textarea on mobile (h-48 sm:h-64)
• Smaller padding on mobile (p-3 sm:p-4)

Keyboard navigation

• Arrow up/down moves through events in the timeline
• Selected event auto-scrolls into view
• Focus container has tabIndex={0} for keyboard focus
• Hint text shows "↑↓ to navigate"

Other polish

• Sticky header for better UX on long traces
• Error state: improved guidance for empty input
• All states (loading, empty, error) properly handled and hidden during loading

Verification

• pnpm lint — ✅
• pnpm typecheck — ✅
• pnpm test — ✅ (196 tests)
• pnpm build — ✅
• pnpm format:check — ✅