fix(spa): back button respects history; auto-redirect / → /hjem when logged in

Two related navigation fixes:

1. Back-button now uses real browser history. Sub-view "Tilbake"
   buttons (permalink, tag page, personvern, public list) all used
   to call goPublicHome() — which always sent the user to / —
   instead of returning to wherever they came from. Replaced with
   a single backToCallerOrHome() that delegates to
   navigate.goBack(fallback), which calls window.history.back() if
   there's a prior entry, else navigates to /hjem (or / when
   anonymous) so cold-loaded permalinks still have somewhere to go.

2. Cold-load redirect: a logged-in user landing on / probably
   wants their own dashboard, not the public marketing surface.
   After the me-probe finishes in onMount, if the initial route
   was 'public-home' and session.user exists, push to /hjem
   instead of applying the public-home route. This only runs on
   initial mount (not on every popstate), so browser-back from
   /hjem to / still works if the user explicitly navigates there.

The wordmark in the nav also picks its destination by auth state
now — logged-in users go to /hjem, anonymous users to /. Otherwise
clicking it post-redirect would just bounce back to /hjem.

frontend/src/App.svelte

@@ -3,6 +3,7 @@
   import { ready } from './lib/crypto';
   import { api, ApiError } from './lib/api';
   import { session, setSessionUserOnly } from './lib/session.svelte';
+  import { goBack } from './lib/navigate';
   import { logout } from './lib/auth';
   import Login from './components/Login.svelte';
   import Signup from './components/Signup.svelte';
@@ -138,7 +139,18 @@
       // No session — fine.
     }
 
-    applyRoute(route);
+    // Cold-load redirect: a logged-in user landing on the public landing
+    // probably wants their own dashboard, not the marketing-y "what is this"
+    // page. We only redirect on this initial mount — not on every
+    // applyRoute call — so browser-back from /hjem to / still lets the
+    // explicit navigation through (no loop, and the wordmark intentionally
+    // sends logged-in users to /hjem instead of / anyway).
+    if (route.view === 'public-home' && session.user) {
+      pushUrl('/hjem');
+      view = 'home';
+    } else {
+      applyRoute(route);
+    }
   });
 
   function applyRoute(route: Route) {
@@ -164,22 +176,20 @@
     }
   }
 
-  function leaveTag() {
-    // Same logic as leavePersonvern — back to wherever they were.
-    if (session.user) goHome();
-    else goPublicHome();
-  }
-
   function goPersonvern() {
     pushUrl('/personvern');
     view = 'personvern';
   }
 
-  function leavePersonvern() {
-    // Send the visitor wherever they "would have been" — landing if logged out,
-    // dashboard if logged in. Either is more useful than staying on the doc page.
-    if (session.user) goHome();
-    else goPublicHome();
+  /**
+   * Back-button handler for sub-views (permalink, tag page, personvern,
+   * public list). Uses real browser history so the user returns to
+   * wherever they came from in the SPA — /hjem, /etiketter/foo,
+   * /aktivitet/bar, anywhere. Falls back to /hjem (or / when anonymous)
+   * on cold-loads where there's no prior history entry.
+   */
+  function backToCallerOrHome() {
+    goBack(session.user ? '/hjem' : '/');
   }
 
   function onAuthed() {
@@ -207,7 +217,8 @@
 <main>
   <nav class="top">
     <h1 style="margin: 0;">
-      <a href="/" onclick={(e) => { e.preventDefault(); goPublicHome(); }}
+      <a href={session.user ? '/hjem' : '/'}
+         onclick={(e) => { e.preventDefault(); session.user ? goHome() : goPublicHome(); }}
          style="color: inherit; text-decoration: none;">Vinterliste</a>
     </h1>
     {#if view !== 'public-list' && view !== 'permalink' && view !== 'tag'}
@@ -246,9 +257,9 @@
   {#if view === 'loading'}
     <p class="muted">Laster …</p>
   {:else if view === 'public-list'}
-    <PublicList username={publicListUsername} onBack={goPublicHome} />
+    <PublicList username={publicListUsername} onBack={backToCallerOrHome} />
   {:else if view === 'permalink'}
-    <ActivityPermalink id={activityId} onBack={goPublicHome} />
+    <ActivityPermalink id={activityId} onBack={backToCallerOrHome} />
   {:else if view === 'public-home'}
     <Home publicOnly={true} />
   {:else if view === 'login'}
@@ -276,9 +287,9 @@
   {:else if view === 'moderate-tags'}
     <ModerateTags onDone={goHome} />
   {:else if view === 'personvern'}
-    <Personvern onBack={leavePersonvern} />
+    <Personvern onBack={backToCallerOrHome} />
   {:else if view === 'tag'}
-    <TagPage tag={tagName} onBack={leaveTag} />
+    <TagPage tag={tagName} onBack={backToCallerOrHome} />
   {:else}
     <Home publicOnly={false} />
   {/if}

frontend/src/lib/navigate.ts

@@ -34,3 +34,21 @@ export function onSpaLink(e: MouseEvent, path: string): void {
   e.preventDefault();
   navigate(path);
 }
+
+/**
+ * "Back" navigation for in-app back buttons. Uses real browser history
+ * when there's prior history to return to (preserves the user's actual
+ * path through the SPA). Falls back to navigate(fallback) on cold-loads
+ * — when the user opened the URL directly in a new tab and there's no
+ * prior entry to back into.
+ *
+ * `window.history.length` is browser-dependent but length === 1 is a
+ * reliable cold-load signal: every browser counts the current entry.
+ */
+export function goBack(fallback: string): void {
+  if (window.history.length > 1) {
+    window.history.back();
+  } else {
+    navigate(fallback);
+  }
+}