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>

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.11.1"
+version = "0.12.0"
 edition = "2021"
 
 [lib]