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/upload/+page.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
     import { goto } from '$app/navigation';
-    import { ApiError } from '$lib/api/client';
+    import { ApiError, fileUrl } from '$lib/api/client';
     import { createManga } from '$lib/api/mangas';
     import { request } from '$lib/api/client';
     import { session } from '$lib/session.svelte';
@@ -74,6 +74,7 @@
     let chapterSuccess = $state<string | null>(null);
     let isDragOver = $state(false);
 
+    const selectedManga = $derived(mangas.find((m) => m.id === chapterMangaId) ?? null);
     const allChapterPagesValid = $derived(chapterPages.every((p) => !p.error));
     const canSubmitChapter = $derived(
         Boolean(chapterMangaId) &&
@@ -276,10 +277,32 @@
                     >
                         <option value="">Choose…</option>
                         {#each mangas as m (m.id)}
-                            <option value={m.id}>{m.title}</option>
+                            <option value={m.id}>
+                                {m.title}{#if m.author} — {m.author}{/if}
+                            </option>
                         {/each}
                     </select>
                 </label>
+                {#if selectedManga}
+                    <div class="manga-preview" data-testid="chapter-manga-preview">
+                        {#if selectedManga.cover_image_path}
+                            <img
+                                src={fileUrl(selectedManga.cover_image_path)}
+                                alt=""
+                                class="preview-cover"
+                                loading="lazy"
+                            />
+                        {:else}
+                            <span class="preview-cover preview-cover-placeholder" aria-hidden="true">📖</span>
+                        {/if}
+                        <div class="preview-meta">
+                            <span class="preview-title">{selectedManga.title}</span>
+                            {#if selectedManga.author}
+                                <span class="preview-author">{selectedManga.author}</span>
+                            {/if}
+                        </div>
+                    </div>
+                {/if}
                 <label>
                     Chapter number <span aria-hidden="true">*</span>
                     <input
@@ -415,6 +438,42 @@
     .success {
         color: #0a7d2c;
     }
+    .manga-preview {
+        display: flex;
+        align-items: center;
+        gap: 0.75rem;
+        padding: 0.5rem;
+        border: 1px solid #eee;
+        border-radius: 4px;
+        background: #fafafa;
+    }
+    .preview-cover {
+        width: 48px;
+        height: 72px;
+        object-fit: cover;
+        border-radius: 3px;
+        background: #f0f0f0;
+        flex-shrink: 0;
+    }
+    .preview-cover-placeholder {
+        display: inline-flex;
+        align-items: center;
+        justify-content: center;
+        font-size: 1.5rem;
+        color: #999;
+    }
+    .preview-meta {
+        display: flex;
+        flex-direction: column;
+        min-width: 0;
+    }
+    .preview-title {
+        font-weight: 600;
+    }
+    .preview-author {
+        color: #777;
+        font-size: 0.85rem;
+    }
     .drop-zone {
         border: 2px dashed #aaa;
         border-radius: 6px;