Batch 2 — Offline Write Queue

Routed the most common user mutations through offlineWrite:

• Reminder dismissals, expense deletes, trip budget, collaborator removal
• Document delete, profile/preferences/language saves, onboarding completion

Users now get a "Will sync when back online" toast instead of a hard error when offline. The header ConnectionBar already shows pending count and auto-flushes on reconnect.

Next: Batch 3 — GPS (battery-light, app-open only).