feat: show manga covers everywhere a manga is referenced

Every place that surfaced a manga title used to show *just* the
title — the home page, the reader's back-to-manga link, and the
chapter upload form's manga selector. Adding the cover image
alongside makes the app feel like an actual manga library.

- Home (`/`): manga list switched from a one-line `<a>` per item to
  a responsive grid of cards (`auto-fill, minmax(140px, 1fr)`),
  each card showing the cover (with 📖 placeholder when no cover is
  set), the title (line-clamped to 2 rows), and the author.
- Reader (`/manga/[id]/chapter/[n]`): the back-to-manga link in the
  reader header now shows a 28×42 thumbnail of the manga's cover next
  to the title. Reuses the placeholder pattern for cover-less mangas.
- Upload (`/upload`): the chapter form's manga `<select>` still uses
  a native dropdown (covers don't fit in `<option>`), but a preview
  pane below the select now shows the currently-selected manga's
  cover + title + author so the user can visually confirm which
  manga they're attaching the chapter to.

No backend changes — `cover_image_path` was already in the Manga
JSON; only the frontend needed to read it.

Lockstep version bump to 0.12.0.

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

frontend/src/routes/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
     import { onMount } from 'svelte';
     import { listMangas, type Manga, type MangaSort } from '$lib/api/mangas';
+    import { fileUrl } from '$lib/api/client';
 
     let mangas: Manga[] = $state([]);
     let search = $state('');
@@ -71,11 +72,25 @@
             Showing {mangas.length} of {total}
         </p>
     {/if}
-    <ul data-testid="manga-list">
+    <ul class="manga-grid" data-testid="manga-list">
         {#each mangas as m (m.id)}
-            <li>
-                <a href="/manga/{m.id}">{m.title}</a>
-                {#if m.author}<span> — {m.author}</span>{/if}
+            <li class="manga-card">
+                <a href="/manga/{m.id}" class="cover-link" aria-hidden="true" tabindex="-1">
+                    {#if m.cover_image_path}
+                        <img
+                            src={fileUrl(m.cover_image_path)}
+                            alt=""
+                            class="cover"
+                            loading="lazy"
+                        />
+                    {:else}
+                        <div class="cover cover-placeholder">📖</div>
+                    {/if}
+                </a>
+                <div class="meta">
+                    <a href="/manga/{m.id}" class="title">{m.title}</a>
+                    {#if m.author}<span class="author">{m.author}</span>{/if}
+                </div>
             </li>
         {/each}
     </ul>
@@ -97,4 +112,58 @@
         color: #777;
         margin: 0.5rem 0;
     }
+    .manga-grid {
+        list-style: none;
+        padding: 0;
+        display: grid;
+        grid-template-columns: repeat(auto-fill, minmax(140px, 1fr));
+        gap: 1rem;
+    }
+    .manga-card {
+        display: flex;
+        flex-direction: column;
+        gap: 0.4rem;
+    }
+    .cover-link {
+        display: block;
+        line-height: 0;
+    }
+    .cover {
+        width: 100%;
+        aspect-ratio: 2 / 3;
+        object-fit: cover;
+        border-radius: 4px;
+        background: #f0f0f0;
+    }
+    .cover-placeholder {
+        display: flex;
+        align-items: center;
+        justify-content: center;
+        font-size: 2.5rem;
+        color: #999;
+        user-select: none;
+    }
+    .meta {
+        display: flex;
+        flex-direction: column;
+        min-width: 0;
+    }
+    .title {
+        font-weight: 600;
+        font-size: 0.95rem;
+        line-height: 1.3;
+        overflow: hidden;
+        display: -webkit-box;
+        -webkit-line-clamp: 2;
+        line-clamp: 2;
+        -webkit-box-orient: vertical;
+    }
+    .author {
+        color: #777;
+        font-size: 0.85rem;
+        margin-top: 0.1rem;
+        white-space: nowrap;
+        overflow: hidden;
+        text-overflow: ellipsis;
+    }
 </style>