docs(web): add web-ui.md reference and update web/README.md

[cursor]

Docs added/updated

docs/web-ui.md (new)

Full developer reference for the React app that shipped in PR #17 but had no dedicated documentation:

• Routing table — HashRouter path → component → HTTP calls
• Component tree — visual diagram of App → AppShell → TimelineRefreshProvider → pages
• TimelineRefreshContext — explains the generation counter pattern, when to call notifyTimelineMutated(), and the end-to-end data flow for post-mutation timeline refresh
• Per-page reference — OverviewPage, DiffPage, ActionsPage: fields, defaults, state management, error handling, auth behavior
• api.ts helpers — fetchJson<T> auth injection / error extraction, loadTimeline() parallel fan-out, all exported TypeScript types
• Shared components — Badge and JsonPanel props/usage
• CSS design tokens — all --fd-* custom properties and key utility classes
• Environment variables — VITE_FLIGHTDECK_LOCAL_API_TOKEN and VITE_DEV_PROXY_TARGET
• Adding a new page — step-by-step guide

web/README.md (updated)

• Removed the stale "PR split (subagent-friendly)" section that described future work (React Router, richer diff, PR B/C) which has since shipped
• Added an App architecture section with the route/component table, shell+context data-flow summary, and a pointer to docs/web-ui.md

Codepaths covered

web/src/App.tsx, web/src/components/AppShell.tsx, web/src/context/TimelineRefreshContext.tsx, web/src/api.ts, web/src/pages/OverviewPage.tsx, web/src/pages/DiffPage.tsx, web/src/pages/ActionsPage.tsx, web/src/components/Badge.tsx, web/src/components/JsonPanel.tsx, web/src/index.css

Key knowledge gaps addressed

• No documentation existed for the React app architecture, routing strategy, or how mutations in ActionsPage trigger automatic refreshes in OverviewPage
• web/README.md contained forward-looking PR-split notes that were stale and potentially confusing for new contributors