feat(reader): chapter select dropdown for direct chapter jumps (0.51.0)

Adds a chapter `<select>` to the reader's top nav listing every chapter
of the current manga, defaulting to the open chapter; picking another
entry navigates straight to it without going back to the manga detail
page. Options use the "Ch. N — Title" form to match the existing
chapter tile and prev/next buttons in the reader bar.

Covered by a new Playwright spec.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/manga/[id]/chapter/[chapter_id]/+page.svelte

@@ -459,6 +459,27 @@
     </a>
 
     <div class="controls" role="group" aria-label="reader options">
+        <label class="chapter-field">
+            <span class="visually-hidden">Jump to chapter</span>
+            <select
+                class="chapter-select"
+                value={chapter.id}
+                onchange={(e) => {
+                    const target = (e.currentTarget as HTMLSelectElement).value;
+                    if (target && target !== chapter.id) {
+                        void goto(`/manga/${manga.id}/chapter/${target}`);
+                    }
+                }}
+                data-testid="reader-chapter-select"
+            >
+                {#each sortedChapters as c (c.id)}
+                    <option value={c.id}>
+                        Ch. {c.number}{c.title ? ` — ${c.title}` : ''}
+                    </option>
+                {/each}
+            </select>
+        </label>
+
         <div class="mode-toggle" role="radiogroup" aria-label="layout">
             <button
                 type="button"
@@ -801,7 +822,8 @@
         outline-offset: -2px;
     }
 
-    .gap-field select {
+    .gap-field select,
+    .chapter-select {
         height: 32px;
         padding: 0 var(--space-2);
         background: var(--surface);
@@ -811,6 +833,13 @@
         font-size: var(--font-sm);
     }
 
+    /* Cap the chapter dropdown's resting width so long titles don't
+       push the rest of the nav off-screen; the native control's
+       expanded menu still shows full option text on focus. */
+    .chapter-select {
+        max-width: 16rem;
+    }
+
     .visually-hidden {
         position: absolute;
         width: 1px;