feat(dashboard): first-visit onboarding overlay

[nieao]

TL;DR

Add a 5-step modal that walks new users through the dashboard's core operations
on first visit. Strictly additive: +282 / -0 lines across one new file and a
6-line lazy mount in App.tsx. Auto-hides via localStorage after dismiss;
?onboard=force re-shows it for screenshots and demos.

Motivation

Landing on the dashboard for the first time, a new user sees a node-link graph,
a search box, three view buttons, a layer dropdown, a tour menu, and a handful
of hidden tools (filter / export / path / theme). The visual is striking, but
"what do I click first" isn't obvious — especially on graphs with 50+ nodes
where the entry point matters.

This adds a one-shot teaching moment that names the three views, points at
search + node click, surfaces the layer/tour pair, and lists the hidden tools.

What it teaches (5 steps)

1. What the graph represents — entities/relations from code or a knowledge wiki
2. Three view buttons (Overview / Learn / Deep Dive) — each answers a
   different question
3. Search + node click — find by name, click for the details panel
4. Layer switch + Project Tour — drill into one category, or follow a
   guided walkthrough
5. Hidden features (Filter / Export / Path / Theme) and Shift+? for
   keyboard shortcuts

Design

• Lazy-loaded via Suspense, mirroring the existing KeyboardShortcutsHelp
  and PathFinderModal mount pattern — ships in its own chunk, no impact on
  initial bundle size for users who've dismissed it.
• Inline styles, no new CSS file — easier to land alongside the existing
  structure without touching the global stylesheet.
• Architectural-minimalism dark palette consistent with the existing
  dashboard: off-black surface, warm accent (#c8a882), Noto Serif SC
  headings, generous whitespace.
• localStorage key ua-onboarding-dismissed-v1 — versioned so future
  content changes can re-trigger when needed.
• Accessible: keyboard-navigable buttons, click-outside to close (without
  remembering dismiss), explicit "不再显示 / Skip" affordance.

Tested on

Windows 11 + Chrome via Playwright CDP. All 5 steps render, the progress bar
tracks, prev / next / dismiss / finish all behave correctly, localStorage
persists dismiss across reloads, ?onboard=force re-shows for testing.

Compatibility

• No new dependencies — uses React 19 hooks already present.
• No changes to data flow, store, schema, or other components — strictly
  additive. If OnboardingOverlay is removed, nothing else breaks.
• First-visit only by default, then permanently dismissed — no impact on
  returning users.

🤖 Generated with Claude Code