Add titles-only list export and Custom lists

Export
- GET /l/<list>/export?format=csv|txt downloads a list's titles in rank
  order, one per line, with no header or other fields (CSV gets a UTF-8
  BOM for Excel)
- Export menu in the list editor and, for the owner, on the public list
  page; hidden lists stay owner-only

Custom lists
- New "Custom" category for anything not built in (sports teams,
  drinks…): owner-named, slug from the title, entries added by hand or
  from an unfiltered Wikipedia search
- Per-list artwork shape (square or tall), stored in a new lists.art
  column (migration 2)
- Art can be uploaded while adding an entry, not only after

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/templates/pages/admin_list.html

@@ -13,6 +13,7 @@
     </div>
     <div class="admin-head-actions">
       <a class="btn btn--ghost" data-bind="public" href="/l/{{.Data.List.Slug}}">{{icon "external"}}View</a>
+      {{template "export" .Data.List.Slug}}
       <button class="btn" type="button" data-action="edit-list">{{icon "settings"}}List settings</button>
     </div>
   </header>
@@ -21,12 +22,12 @@
   <div class="editor">
     <section class="panel editor-search" aria-labelledby="add-h">
       <div class="panel-head">
-        <h2 id="add-h">Add {{$c.Label}}</h2>
+        <h2 id="add-h">{{if eq $c.Key "custom"}}Add favorites{{else}}Add {{$c.Label}}{{end}}</h2>
         <span class="muted small" data-bind="provider"></span>
       </div>
       <form class="search-bar" data-search role="search">
         {{icon "search"}}
-        <input type="search" name="q" placeholder="Search for a {{$c.Singular}}…" autocomplete="off" aria-label="Search">
+        <input type="search" name="q" placeholder="{{if eq $c.Key "custom"}}Search Wikipedia (optional)…{{else}}Search for a {{$c.Singular}}…{{end}}" autocomplete="off" aria-label="Search">
         <span class="spinner" hidden></span>
       </form>
       <div class="search-results" data-results aria-live="polite"></div>