Surface failure states across client (toasts, error-vs-empty, no-access)

[Alimedhat000]

Problems

• Rename/delete failures are console-only with literal TODOs (rename-modal.tsx:47-49, delete-modal.tsx:31-33); modal stays open, row unchanged, user gets no signal.
• New-document creation failures console-only (new-document-modal.tsx:21-24).
• Dashboard fetch failure renders the "It's empty in here…" empty state (dashboard.tsx:19-23) — a failed load masquerades as an empty workspace.
• Editor silently redirects to dashboard when doc is missing/no-permission (document.tsx:28-31); permission union also mismatched (server returns 'owner' | 'none', client types only 'view' | 'edit').

Related: #29 (404 page), #30 (error boundary), #32 (blank loading screen).