Settings store (persistent serialization)

[Phoenix-]

Persistent settings backend, distinct from the UI that will sit on top of it.

Today user-toggleable values are parked as const in AppSettings because there is no store. As soon as 3-4 real toggles accumulate this becomes the blocker for further user-facing work (Agenda view options, calendar selection, etc.).

Scope

• Decide store format & location (JSON under %APPDATA%\Meridian\, probably).
• Serialization that survives NativeAOT (source-gen, no reflection).
• Change notifications so views can react.
• Migration story for future schema changes (version field + handlers).
• No UI in this issue — just the API.

Out of scope

• The Settings View itself (separate issue).
• Per-account settings (assume single-user for now).