refactor(frontend): one account per browser, its local state under plain keys

The per-tab account pin, the X-Siftlode-Account header and ?account= go: the session cookie alone
says who is asking. localStorage keys lose their account suffix; the browser records whose state it
holds (siftlode.owner), BootGate hands it to the account the server answered as, and a different
account starts from a clean slate (the public /watch keys stay). The first boot moves the account's
old per-account copies onto the plain keys and drops other accounts' copies. Until the claim, and
after another tab claimed the browser for someone else, the account helpers read and write nothing.

The account panel lists no other accounts: "Add another account" becomes "Sign in as someone else"
(Google's account chooser; the pick replaces this account, a cancel keeps it), shown where Google
sign-in is configured.

frontend/src/lib/api/diagnostics.ts

@@ -2,7 +2,7 @@
 // beacon. The streaming itself — what is sent, when, and when it stops — is `lib/diagnostics.ts`.
 import type { LogEvent } from "../log";
 import type { ServerLogRow } from "./admin";
-import { beacon, getActiveAccount, identityHeaders, req } from "./core";
+import { beacon, identityHeaders, req } from "./core";
 
 /** Alap / Részletes / Teljes. The server decides how long a raised level lasts. */
 export type DiagnosticsLevel = "INFO" | "DEBUG" | "TRACE";
@@ -123,12 +123,7 @@ export const debugSessionsApi = {
     req(`${SESSIONS}/${id}/preview`, { method: "POST", body: JSON.stringify({ text }) }),
   removeDebugScreenshot: (id: string, shotId: number): Promise<{ ok: true; deleted: number }> =>
     req(`${SESSIONS}/${id}/screenshots/${shotId}`, { method: "DELETE" }),
-  /** Where an `<img>` loads a screenshot from. An `<img>` cannot send the account header, so the
-   *  tab's account rides as `?account=` (the wallet-gated selection downloads use); without it a
-   *  per-tab account's screenshot resolves to the session default and 404s. */
-  debugScreenshotUrl: (id: string, shotId: number): string => {
-    const a = getActiveAccount();
-    const url = `${SESSIONS}/${id}/screenshots/${shotId}`;
-    return a != null ? `${url}?account=${a}` : url;
-  },
+  /** Where an `<img>` loads a screenshot from. */
+  debugScreenshotUrl: (id: string, shotId: number): string =>
+    `${SESSIONS}/${id}/screenshots/${shotId}`,
 };