Guard localStorage access against SecurityError in restricted browsers

[iHiD]

Closes #8595

Summary

• Added a safeLocalStorage() helper that wraps window.localStorage in a try/catch and returns null when access is denied (Safari private browsing, restricted iframes, strict cookie settings)
• Protected the React Query persister setup in application.tsx — the library already accepts null and returns a no-op persister, so the app gracefully skips cache persistence
• Protected the sign-out localStorage cleanup in application.tsx — if storage is inaccessible, there's nothing to clear
• Wrapped direct localStorage.getItem() calls in NewListItemForm.tsx and StartDiscussionPanel.tsx with try/catch, falling back to empty string
• Added a Sentry beforeSend filter to suppress the non-actionable SecurityError for localStorage/sessionStorage access, following the existing pattern for other browser-specific errors

Test plan

• yarn test passes (1550 tests passed)
• Pre-commit hooks pass (prettier auto-formatted)
• Verified createSyncStoragePersister gracefully handles null storage (returns no-op persister)
• Sentry filter pattern matches the exact error message from the issue

🤖 Generated with Claude Code