feat(web): copy-link button on owned public playlists (Tier C12)

Add a Link icon button to the playlist header that copies the
absolute /playlists/[id] URL to the clipboard. Visible only when
isOwner && pl.is_public. Falls back to a window.prompt() in
insecure contexts where navigator.clipboard is unavailable.

Tests: button visibility (private vs public owner) + clipboard
write target.

web/src/routes/playlists/[id]/+page.svelte

@@ -2,7 +2,7 @@
   import { page } from '$app/state';
   import { goto } from '$app/navigation';
   import { pageTitle } from '$lib/branding';
-  import { Pencil, Trash2 } from 'lucide-svelte';
+  import { Pencil, Trash2, Link as LinkIcon } from 'lucide-svelte';
   import { useQueryClient } from '@tanstack/svelte-query';
   import PlaylistTrackRow from '$lib/components/PlaylistTrackRow.svelte';
   import ApiErrorBanner from '$lib/components/ApiErrorBanner.svelte';
@@ -130,6 +130,18 @@
     }
   }
 
+  async function onCopyLink() {
+    const href = `${window.location.origin}/playlists/${id}`;
+    try {
+      await navigator.clipboard.writeText(href);
+      pushToast('Link copied to clipboard.');
+    } catch {
+      // navigator.clipboard fails in insecure contexts and older browsers.
+      // Fall back to selecting the link in a prompt so the user can copy.
+      window.prompt('Copy the playlist link:', href);
+    }
+  }
+
   // --- System-playlist refresh (#411 R2: generic by-kind) ---
   let refreshingSystem = $state(false);
 
@@ -188,6 +200,17 @@
             {refreshingSystem ? 'Refreshing…' : 'Refresh'}
           </button>
         {/if}
+        {#if isOwner && pl.is_public}
+          <button
+            type="button"
+            onclick={onCopyLink}
+            aria-label="Copy public link to this playlist"
+            title="Copy link"
+            class="rounded p-2 text-text-muted hover:bg-surface-hover hover:text-text-primary"
+          >
+            <LinkIcon size={16} strokeWidth={1} />
+          </button>
+        {/if}
         {#if isOwner}
           <button
             type="button"