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/manga/[id]/chapter/[n]/+page.svelte

@@ -70,7 +70,19 @@
 </svelte:head>
 
 <nav class="reader-nav" aria-label="reader">
-    <a href="/manga/{manga.id}" data-testid="back-to-manga">← {manga.title}</a>
+    <a href="/manga/{manga.id}" class="back" data-testid="back-to-manga">
+        {#if manga.cover_image_path}
+            <img
+                src={fileUrl(manga.cover_image_path)}
+                alt=""
+                class="back-cover"
+                loading="lazy"
+            />
+        {:else}
+            <span class="back-cover back-cover-placeholder" aria-hidden="true">📖</span>
+        {/if}
+        <span class="back-text">← {manga.title}</span>
+    </a>
     <span class="indicator" data-testid="page-indicator">
         Page {index + 1} / {pages.length}
     </span>
@@ -133,9 +145,37 @@
         padding-bottom: 0.5rem;
         border-bottom: 1px solid #eee;
         margin-bottom: 0.75rem;
+        gap: 0.75rem;
+    }
+    .back {
+        display: flex;
+        align-items: center;
+        gap: 0.5rem;
+        min-width: 0;
+    }
+    .back-cover {
+        width: 28px;
+        height: 42px;
+        object-fit: cover;
+        border-radius: 3px;
+        background: #f0f0f0;
+        flex-shrink: 0;
+    }
+    .back-cover-placeholder {
+        display: inline-flex;
+        align-items: center;
+        justify-content: center;
+        font-size: 1rem;
+        color: #999;
+    }
+    .back-text {
+        overflow: hidden;
+        text-overflow: ellipsis;
+        white-space: nowrap;
     }
     .indicator {
         color: #555;
+        flex-shrink: 0;
     }
     .page-wrap {
         display: grid;