Fix dashboard crash when localStorage is unavailable

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Bug and impact

Browsers that throw on localStorage access (for example restricted/private profiles, blocked site storage, or quota/security failures) could successfully fetch and render a snapshot, then throw during post-render persistence. The outer loadSnapshot catch treated that persistence failure as a load failure, called showError(), and cleared the populated qualified table, leaving the dashboard unusable despite valid data.

Root cause

docs/dashboard/app.js mixed critical render/load logic with direct localStorage.getItem/setItem/removeItem calls. Several of those calls run during startup and after render(data), so a storage exception could abort the page or unwind into the snapshot load error handler.

Fix

Added small safe localStorage helpers and routed dashboard storage reads/writes/removals through them. Storage-backed preferences and notification baselines now degrade gracefully while valid snapshot rendering remains intact.

Validation

• node --check docs/dashboard/app.js
• CDP browser test against the real dashboard with a pre-load shim that makes window.localStorage throw SecurityError: rendered 41 table rows, empty error banner, populated snapshot metadata.
• Manual GUI walkthrough against http://127.0.0.1:8765/dashboard/?api=../qualified_public_snapshot.json confirmed populated table rows and no visible error banner; saved video artifact dashboard_storage_fix_manual_walkthrough.mp4.