Synced app settings: colour theme and appearance (beta)

One encrypted settings item per account (a secure note carrying
app_settings, at a fixed per-account id) holds the theme and light/dark
appearance, so it syncs like any item and the server can't read it. Lists
hide it; older apps see a note that explains itself. app_theme /
set_app_theme in kv-ffi and kv-wasm. Also fixes a flaky test that checked
random ciphertext for the letters K1.

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

crates/kv-wasm/src/wasm/mod.rs

@@ -51,6 +51,9 @@ export interface SignInWith { provider: string; account?: string | null; }
 
 export interface SignInProvider { id: string; name: string; }
 
+/** Synced theme (docs/02 "App settings"): a theme id such as "ocean", and "system" | "light" | "dark". */
+export interface AppThemeDto { theme: string | null; appearance: string | null; }
+
 /** `id: ""` creates a new item. Omitted optional fields default to null / [] / false / 0. */
 export interface ItemDto {
   id: string;
@@ -525,6 +528,17 @@ impl VaultCore {
         self.run(|s| async move { s.core.list_items() })
     }
 
+    /// The colour theme and appearance synced across this account's devices.
+    #[wasm_bindgen(js_name = appTheme, unchecked_return_type = "Promise<AppThemeDto>")]
+    pub fn app_theme(&self) -> Promise {
+        self.run(|s| async move { s.core.app_theme() })
+    }
+
+    #[wasm_bindgen(js_name = setAppTheme, unchecked_return_type = "Promise<void>")]
+    pub fn set_app_theme(&self, theme: Option<String>, appearance: Option<String>) -> Promise {
+        self.run(move |s| async move { s.core.set_app_theme(theme, appearance) })
+    }
+
     #[wasm_bindgen(js_name = getItem, unchecked_return_type = "Promise<ItemDto>")]
     pub fn get_item(&self, id: String) -> Promise {
         self.run(move |s| async move { s.core.get_item(&id) })