feat(auth): per-tab account — different account per browser tab

Two tabs in one browser can now run two different signed-in accounts at once.

- The signed session cookie stays the browser's account WALLET (account_ids). Which account a
  given tab acts as is a per-tab choice held in sessionStorage and sent per-request via the
  X-Siftlode-Account header; current_user honours it only for an account already in the wallet,
  without mutating the cookie's default account. Switching accounts sets the header + reloads
  THIS tab only, instead of the old cookie-wide switch that changed every tab.
- WebSocket can't send headers, so the per-tab account rides in the ?account= query param
  (validated against the wallet).
- Logout is per-tab aware: it signs the requesting tab's active account out of the wallet
  (promoting a new default only if the removed one was the default), and the tab drops its
  override. A stale per-tab header account 401s just that tab instead of clearing the session.
- Serve index.html with Cache-Control: no-cache so a deploy's new hashed bundle is picked up
  immediately instead of the browser running a heuristically-cached stale index.html.

frontend/src/components/NavSidebar.tsx

@@ -20,7 +20,7 @@ import {
   Tv,
   UserPlus,
 } from "lucide-react";
-import { api, type Me } from "../lib/api";
+import { api, clearActiveAccount, setActiveAccount, type Me } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { getUnreadCount, subscribe } from "../lib/notifications";
 import type { Page } from "../lib/urlState";
@@ -94,7 +94,14 @@ export default function NavSidebar({
   }, [acctOpen]);
 
   async function logout() {
-    await fetch("/auth/logout", { method: "POST", credentials: "include" });
+    // Signs THIS tab's active account out of the browser wallet (server is per-tab aware); then
+    // drops this tab's override and reloads onto whatever account remains the default.
+    try {
+      await api.logout();
+    } catch {
+      /* clear locally and reload regardless */
+    }
+    clearActiveAccount();
     location.reload();
   }
 
@@ -106,21 +113,20 @@ export default function NavSidebar({
   });
   const otherAccounts = (accountsQuery.data ?? []).filter((a) => !a.active);
 
-  async function switchTo(id: number) {
-    try {
-      await api.switchAccount(id);
-      // Drop the previous account's in-module sub-view/overlay before the reload (which would
-      // otherwise preserve history.state across the swap). Otherwise the new account lands on a
-      // stale sub-view — e.g. an open chat thread whose partnerId belongs to the OLD identity
-      // (often the new account's own id → an empty self-thread). Start at the module root instead.
-      const st = { ...(window.history.state || {}) };
-      delete st._sub;
-      delete st._ov;
-      window.history.replaceState(st, "");
-      location.reload(); // cleanest way to reload all per-user state for the new account
-    } catch {
-      /* a revoked/removed account — leave the popover open */
-    }
+  function switchTo(id: number) {
+    // Per-tab switch: point THIS tab at the account and reload; other tabs keep their own
+    // identity. No server round-trip — req() sends the account header (validated against the
+    // browser wallet server-side), so the cookie's default account is left untouched.
+    setActiveAccount(id);
+    // Drop the previous account's in-module sub-view/overlay before the reload (which would
+    // otherwise preserve history.state across the swap). Otherwise the new account lands on a
+    // stale sub-view — e.g. an open chat thread whose partnerId belongs to the OLD identity
+    // (often the new account's own id → an empty self-thread). Start at the module root instead.
+    const st = { ...(window.history.state || {}) };
+    delete st._sub;
+    delete st._ov;
+    window.history.replaceState(st, "");
+    location.reload(); // cleanest way to reload all per-user state for the new account
   }
 
   // Durable, server-backed unread count for the inbox badge. Polled live (pauses when the