refactor(catalog): derive desktop sort options from SORT_FIELD_LABELS

The desktop sort <select> hard-coded its four <option>s while the mobile
sort sheet rendered them from SORT_FIELD_LABELS. The two happened to match,
but a label rename in mangaSort.ts would have silently diverged the desktop
control. Render the desktop options from the same map so they can't.

No behaviour change (the rendered options are identical), so no version
bump. Adds an e2e guard asserting the desktop select's option values and
labels equal SORT_FIELD_LABELS in order — verified red against the old
hard-coded markup (with a label renamed) and green after.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -538,10 +538,11 @@
         <label class="sort">
             <span>Sort</span>
             <select bind:value={sort} onchange={onSortChange} data-testid="sort-select">
-                <option value="updated">Last updated</option>
-                <option value="created">Date added</option>
-                <option value="title">Title</option>
-                <option value="author">Author</option>
+                <!-- Options derive from SORT_FIELD_LABELS — the same source the
+                     mobile sort sheet uses — so the two surfaces can't drift. -->
+                {#each Object.entries(SORT_FIELD_LABELS) as [value, label] (value)}
+                    <option {value}>{label}</option>
+                {/each}
             </select>
         </label>
         <div class="sort">