refactor(frontend): every stored key declares its kind, and preferences follow the account

storage.ts's registry gives each key a kind (pref, device, view, cache,
public); a key without one does not compile. A contract test refuses a key
spelled out elsewhere, an account preference read or written straight from
storage instead of through usePref, and an e2e seed of a key nobody reads.
The keys the next account's wipe keeps are the public ones.

The seven preferences kept only in the browser move onto usePref: start on
Home, the onboarding dismissal, the release banner's last version, the
channel manager's layout, the Plex playlist layout, the playlist rail's sort
and channel discovery's sort. usePref reads an older bare-string copy, and a
value only this browser held is saved to the server once, so the account's
other devices get it.

Performance mode is the device's: it is neither read from the account nor
saved to it.

frontend/playwright.config.ts

@@ -18,9 +18,10 @@ import { defineConfig, devices } from "@playwright/test";
 const BASE_URL = process.env.PW_BASE_URL || "http://127.0.0.1:8080";
 const STORAGE_STATE = "e2e/.auth/state.json";
 // The axe scan reads the DOM and the accessibility tree, which the engines build alike, and the
-// member spec checks which pages and requests a role gets, which no engine changes: one engine
-// answers for all, so the other projects leave them out instead of tripling their time.
-const CHROMIUM_ONLY = /(a11y|member)\.spec\.ts/;
+// member spec checks which pages and requests a role gets, which no engine changes, and the devices
+// spec opens its own desktop and iPad contexts: one engine answers for all, so the other projects
+// leave them out instead of tripling their time.
+const CHROMIUM_ONLY = /(a11y|member|devices)\.spec\.ts/;
 
 export default defineConfig({
   testDir: "./e2e",