Sign in with Google, Apple…: item field, bridge save and identities

Items record sign_in_with {provider, account}; the browser bridge reports it
with the site's identities and saves it after the usual app prompt. Items
now keep fields they don't know, so editing in an older app never drops what
a newer one added.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/kv-wasm/src/dto.rs

@@ -44,6 +44,8 @@ pub struct ItemSummary {
     pub has_totp: bool,
     /// Web only (not in kv-ffi's record): for tag filters and "recently modified" without `getItem`.
     pub tags: Vec<String>,
+    /// "Sign in with Google…" for the site (docs/05).
+    pub sign_in_with: Option<kv_model::SignInWith>,
     pub modified_at: i64,
 }
 
@@ -70,6 +72,8 @@ pub struct ItemDto {
     #[serde(default)]
     pub tags: Vec<String>,
     #[serde(default)]
+    pub sign_in_with: Option<kv_model::SignInWith>,
+    #[serde(default)]
     pub modified_at: i64,
 }