fix(profile): stop falling back to email when display_name is empty

The owner attribution helper used to fall back from display_name to
the part of the email before "@" when no display name was set. That
defeats the point of letting users pick a name: anyone who hadn't
explicitly chosen one had their email prefix surfaced publicly.

New fallback chain, applied uniformly:
  - display_name (the user's chosen name) — if set, use it
  - username (also chosen by the user as a URL slug) — if set, use it
  - null — render nothing; the client hides the attribution line

Wire type ActivityPublic.owner_display is now `string | null`.
ActivityRow renders the "Lagt til av X" line only when display is
non-null.

Same idea applied to the user's own surfaces (nav + greeting):
  - Nav button shows "Profil" (a label, not a name) when display_name
    is empty, instead of falling back to the email.
  - Home greeting drops "Velkommen, <name>." entirely when the user
    has no display name, leaving just "Her er aktivitetene dine ...".

The feedback list (moderator view) and admin user table keep showing
the email — moderators and admins legitimately need it to identify
users for triage and role management.

frontend/src/App.svelte

@@ -168,7 +168,7 @@
           </button>
           <button type="button" onclick={() => (view = 'profile')}
                   aria-label="Profil">
-            {session.user.display_name?.trim() || session.user.email}
+            {session.user.display_name?.trim() || 'Profil'}
           </button>
           <button onclick={onLogout}>Logg ut</button>
         {:else if view !== 'login' && view !== 'signup' && view !== 'recovery'}

frontend/src/components/ActivityRow.svelte

@@ -211,7 +211,7 @@
       {@render locationLine(activity.loc_label, activity.loc_lat, activity.loc_lng)}
     {/if}
     {#if activity.scheduled_at}<p class="muted">🕒 {formatDate(activity.scheduled_at)}</p>{/if}
-    {#if activity.visibility === 'public'}
+    {#if activity.visibility === 'public' && activity.owner_display}
       <p class="muted" style="font-size: 0.8rem;">
         Lagt til av
         {#if activity.owner_username}

frontend/src/components/Home.svelte

@@ -117,8 +117,11 @@
       </p>
     {:else if session.user}
       <p class="muted" style="margin: 0;">
-        Velkommen, {session.user.display_name?.trim() || session.user.email}.
-        Her er aktivitetene dine for vinteren.
+        {#if session.user.display_name?.trim()}
+          Velkommen, {session.user.display_name}. Her er aktivitetene dine for vinteren.
+        {:else}
+          Her er aktivitetene dine for vinteren.
+        {/if}
       </p>
       {#if !showForm && !editing}
         <button class="primary" onclick={() => (showForm = true)}>Ny aktivitet</button>