Frontend lacks an Error Boundary and guards on untrusted JSON.parse/localStorage #13

Contexte

A few frontend code paths trust input they do not control: JSON.parse on drag-and-drop payloads, on a story.json read from an uploaded ZIP and on an XHR response body; localStorage.getItem in the initial Redux state (throws in some private-browsing modes); and there is no React Error Boundary, so a single render error blanks the whole UI.

Description

Add small defensive guards, no behaviour change on valid input:

• an ErrorBoundary component around the app root that shows a minimal "something went wrong" message instead of a blank page;
• try/catch around each untrusted JSON.parse, ignoring/logging the bad payload;
• a safe localStorage read helper used by the settings reducer.

Fichiers concernes

• web-ui/javascript/src/components/ErrorBoundary.js (new) + web-ui/javascript/src/index.js — wrap the root
• web-ui/javascript/src/components/PackLibrary.js:90, 281 — drag-and-drop JSON.parse
• web-ui/javascript/src/components/diagram/widgets/PackDiagramWidget.js:149 — drag-and-drop JSON.parse
• web-ui/javascript/src/utils/reader.js:25 — JSON.parse of story.json from uploaded ZIP
• web-ui/javascript/src/services/library.js:34-35 — JSON.parse of XHR response (also drop the stray console.log)
• web-ui/javascript/src/utils/storage.js (new) — safeGetItem(key) returning null on throw
• web-ui/javascript/src/reducers/settings.js:14-15 and App.js:145 — use safeGetItem
• Tests: ErrorBoundary.test.js, storage.test.js, and a case in the reader test for a malformed story.json

Plan d'implementation

1. ErrorBoundary (class component with componentDidCatch) + wrap in index.js.
2. utils/storage.js + use in settings.js and App.js.
3. Guard the five JSON.parse sites.
4. Tests for the boundary, the storage helper and the malformed ZIP case.
5. yarn test green.

Criteres d'acceptation

• A thrown render error shows the fallback instead of a blank page (tested)
• Malformed drag payload / story.json / XHR body no longer throws out of the handler (tested for reader.js)
• localStorage throwing during store creation no longer crashes startup (tested)
• yarn test green on Ubuntu and Windows CI

Complexite

S