feat: safe mode for iOS

[datlechin]

Summary

• 3-level safe mode: Off, Confirm Writes, Read-Only
• Per-connection setting via Picker in connection form
• Enforcement across all write surfaces: data browser, row detail, query editor
• Toolbar indicator: lock (red) for read-only, shield (orange) for confirm writes
• Write query detection blocks INSERT/UPDATE/DELETE/DROP/ALTER/CREATE/TRUNCATE
• Backward compatible: existing isReadOnly=true connections become .readOnly

Test plan

• Set "Off" — all operations work normally
• Set "Confirm Writes" — delete/insert show confirmation, SELECT runs freely
• Set "Confirm Writes" — write queries show confirmation dialog before executing
• Set "Read-Only" — insert/delete buttons hidden, Edit hidden in row detail
• Set "Read-Only" — write queries blocked with alert in query editor
• Lock/shield icon appears in toolbar
• Edit existing connection with isReadOnly=true — shows as Read-Only
• iCloud sync preserves safeModeLevel