feat(profiles): switch profile from the account menu

Switching was reachable only from Settings, which is the wrong place for
it: on a shared device changing who is watching is a frequent, front-door
action, not a configuration change buried three screens deep.

It sits directly under the identity block rather than among the
destinations (Downloads/Settings/Display), because it answers "who is
this?" and not "where do I go?". Shown even with one account, since the
picker is also where a second is added -- gating it on a second profile
existing would leave no way in from here.

The picker also gains a Back affordance when a session is already live.
Reaching it from the menu and changing your mind -- or failing a PIN on
someone else's tile -- previously had no way back to the session you
still had. At startup there is nothing behind it, so it stays hidden.

src/routes/profiles/+page.svelte

@@ -13,6 +13,8 @@
   import { onMount } from "svelte";
   import { goto } from "$app/navigation";
   import { profiles } from "$lib/stores/profiles";
+  import { isAuthenticated } from "$lib/stores/auth";
+  import { navigateBack } from "$lib/utils/navigation";
   import type { Profile, UnlockOutcome } from "$lib/api/bindings";
   import PinPad from "$lib/components/PinPad.svelte";
   import { orderProfiles, initialsFor, tileColour, lockoutMessage } from "$lib/utils/profileTiles";
@@ -152,6 +154,30 @@
 
 <div class="min-h-full flex items-center justify-center p-6">
   <div class="w-full max-w-3xl">
+    <!--
+      Reached from the account menu, the picker must not be a one-way door: a
+      signed-in viewer who opens it and changes their mind (or fails a PIN on
+      someone else's tile) needs a way back to the session they still have. At
+      startup there is no session behind it, so there is nothing to go back to
+      and this stays hidden. (DR-276)
+    -->
+    {#if $isAuthenticated && mode !== "pin" && mode !== "password"}
+      <button
+        type="button"
+        onclick={() => navigateBack("/")}
+        class="mb-6 text-sm text-gray-400 hover:text-white flex items-center gap-1"
+      >
+        <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+          <path
+            stroke-linecap="round"
+            stroke-linejoin="round"
+            stroke-width="2"
+            d="M15 19l-7-7 7-7"
+          />
+        </svg>
+        Back
+      </button>
+    {/if}
     {#if mode === "picker" || mode === "manage"}
       <h1 class="text-3xl font-semibold text-center mb-2">
         {mode === "manage" ? "Manage profiles" : "Who's watching?"}