Add settings for what new items start with

Let people choose what new items start with, so they don't have to set the same options every time they add an item.

The settings:

• "Lock New Items"
• "Show New Codes in QuickType" (today every new code starts with showInQuickType = true)

Decide the defaults with MANIFESTO.md C7 in mind, which names autofill restrictions as a protection that should default to on.

Scope:

• These only set the starting values when an item is created. Each item's editor can still change them, and existing items are never touched. That keeps it clear of bulk changes to safety-critical fields (C1).
• Don't add defaults for killphrases, search passphrases or visibility (C1, C9). Hidden visibility needs its own passphrase per item anyway.
• Recovery phrases stay always locked, whatever these say.

Tests: unit tests that the create flows pick up each default, and that editing an existing item ignores them.