test(desktop): story-cover the renderer crash fallback

[liuxiaocs7]

Summary

The top-level renderer ErrorBoundary (apps/desktop/src/renderer/error-boundary.tsx) had no Storybook coverage — it is the surface hardest to reach in normal use and the one whose correctness matters most when reached, so its four copyState values and the repeat-error path could only be seen by actually crashing the app.

This PR extracts the fallback face into a pure presentational ErrorBoundaryFallback component (the class still owns all state and side effects, so behavior is unchanged), then adds one story per state — default fallback, copy pending, copied, copy failed, repeat error — each with a // Real path: annotation per stories/FIDELITY.md. Rendering the fallback directly rather than triggering a real throw keeps the render smoke free of the console.error that componentDidCatch would otherwise emit.

Fixes #3943

Verification

Ran locally, all passing:

• biome lint — clean
• tsc (tsconfig.storybook.json + tsconfig.renderer.json)
• npm run build-storybook — all five product-shell-error-boundary--* in the index
• npm run smoke:storybook — Storybook render smoke passed (200 stories)., all five new stories render clean

The five states are visible in Storybook under Product/Shell/Error Boundary; the render smoke above is the command-output evidence that each mounts without runtime, console, or AX errors.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code assisted with the ErrorBoundaryFallback extraction and the story authoring; the design decisions and verification are my own. The affected commit carries a Generated-by: Claude Code trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No