fix(auth): a tab acts only as the account it was built for, and the review's other findings

A tab names the account it was claimed for on every request, beacon and upload
(X-Siftlode-Expect-Account); current_user and /auth/logout refuse a mismatch with
401 account_changed and keep the session, so a tab left showing the account
signed in before reloads instead of writing to the new one.

/auth/login clears a connect marker left by an abandoned connect, and a link
completes only on a session that still holds. The prefs merge reads the row
under a lock. A tab's sessionStorage records its own owner. A page the account
loses is given up, address bar and remembered page included. The panel layouts
are preferences; the storage contract also refuses a key handed to the Storage
API or to usePref outside its kind. A refused sign-in as someone else and a
failed sign-out say so. The chromium-only e2e specs are in the gate register,
the member walk waits for its requests, and devices.spec checks the save itself.

frontend/src/lib/localData.dom.test.ts

@@ -33,7 +33,7 @@ describe("claiming the browser's local data", () => {
     expect(keys(localStorage)).toEqual(
       ["siftlode.filters", "siftlode.onboarding.dismissed", "siftlode.owner"].sort(),
     );
-    expect(keys(sessionStorage)).toEqual(["siftlode.settingsTab"]);
+    expect(keys(sessionStorage)).toEqual(["siftlode.owner", "siftlode.settingsTab"]);
   });
 
   it("an old per-account copy wins over a plain key already there", async () => {
@@ -82,7 +82,23 @@ describe("claiming the browser's local data", () => {
       ["siftlode.owner", "siftlode.watchRepeat", "siftlode.watchResume", "unrelated.key"].sort(),
     );
     expect(localStorage.getItem("siftlode.owner")).toBe("1");
-    expect(keys(sessionStorage)).toEqual([]);
+    expect(keys(sessionStorage)).toEqual(["siftlode.owner"]);
+  });
+
+  it("a tab reloading for the account another tab claimed the browser for drops its own view state", async () => {
+    // Tab 2 signed in as 7 and claimed the browser; this tab still holds 1's sub-tab and the
+    // onboarding flow 1 was in. Its sessionStorage is its own, so the browser's owner says nothing
+    // about it.
+    localStorage.setItem("siftlode.owner", "7");
+    sessionStorage.setItem("siftlode.owner", "1");
+    sessionStorage.setItem("siftlode.settingsTab", "plex");
+    sessionStorage.setItem("siftlode.onboarding.active", "1");
+    const { claimLocalData } = await storage();
+
+    claimLocalData(7);
+
+    expect(keys(sessionStorage)).toEqual(["siftlode.owner"]);
+    expect(sessionStorage.getItem("siftlode.owner")).toBe("7");
   });
 
   it("the same account starting again keeps everything", async () => {