fix(a11y): use h2 in PublicList to avoid duplicate h1

App.svelte already renders <h1>Vinterliste</h1> in the nav, so the
PublicList page's <h1>{displayName || username}</h1> produced a
two-h1 document — screen-reader users navigating by headings would
see two top-level entries that aren't actually peers.

Switched to <h2>, kept the larger font size inline so the visual
hierarchy is unchanged. Every other view (Home/Profile/Admin/Feedback)
already follows the "nav h1, view h2" pattern; this brings PublicList
in line.

Surfaced by /audit a11y (semantic structure lens).

frontend/src/components/PublicList.svelte

@@ -49,7 +49,7 @@
   {:else if error}
     <p class="error">{error}</p>
   {:else}
-    <h1>{displayName?.trim() || username}</h1>
+    <h2 style="font-size: 1.75rem; margin-top: 0.5rem;">{displayName?.trim() || username}</h2>
     <p class="muted">Offentlige vinteraktiviteter</p>
     {#if !activities.length}
       <p class="muted">Ingen offentlige aktiviteter ennå.</p>