feat: design system with light/dark themes and icon-first UI

Adds a real design system to replace the per-route ad-hoc styling:

- docs/design-system.md is the contract. Semantic CSS custom-property
  tokens (color/type/spacing/radii/shadows/z-index) with verified WCAG
  AA/AAA contrast ratios for both themes.
- frontend/src/lib/styles/tokens.css defines :root tokens + a
  [data-theme="dark"] override + base element resets, a .form-field
  helper, and a global prefers-reduced-motion rule.
- frontend/src/lib/theme.svelte.ts is a Svelte 5 runes store backing
  the theme state machine (system | light | dark). localStorage key
  'mangalord-theme'; matchMedia subscription that re-resolves on OS
  theme change while in 'system' mode; init() / destroy() lifecycle
  wired from +layout.svelte.
- frontend/src/app.html runs a synchronous inline script before
  %sveltekit.head% to set [data-theme] before first paint. No FOUC.
- /settings gains a System / Light / Dark radiogroup (real fieldset +
  legend + radios with lucide icons).
- Every route's <style> block is rewritten to consume tokens — home,
  auth, upload (drop-zone + page list), bookmarks, manga overview,
  reader.
- @lucide/svelte icons replace ad-hoc text controls per the spec:
  Search (icon-only primary), LogOut (icon-only muted), Upload /
  Bookmarks / Settings nav inline icons, ChevronLeft/Right for the
  reader, ArrowUp/Down/Trash2 for the upload page list. The bookmark
  toggle keeps its '☆ Bookmark' / '★ Bookmarked' text verbatim.
- Home search controls split into two rows: input + Search CTA on
  row 1, Sort (and future filters) on row 2.

Accessibility: every icon-only button carries aria-label, every
decorative SVG aria-hidden; existing image alt text preserved;
focus-visible rings reach every interactive element including the
visually-hidden theme radios; color is never the sole conveyor.

Version bump 0.12.0 → 0.13.0 across backend/Cargo.toml and
frontend/package.json (feat: → minor per CLAUDE.md).

Bars: svelte-check 0/0, vitest 51/51, playwright 18/18, cargo test
88/88, clippy -D warnings clean. Two rounds of independent review;
verdict ship-ready.

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

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

@@ -105,53 +105,79 @@
     .overview {
         display: grid;
         grid-template-columns: minmax(0, 200px) 1fr;
-        gap: 1rem;
+        gap: var(--space-4);
         align-items: start;
+        margin-bottom: var(--space-6);
     }
+
     @media (max-width: 640px) {
         .overview {
             grid-template-columns: 1fr;
         }
     }
+
     .cover {
         width: 100%;
         height: auto;
-        border-radius: 4px;
-    }
-    .meta h1 {
-        margin: 0 0 0.5rem;
+        border-radius: var(--radius-md);
+        background: var(--surface);
     }
+
     .author {
-        color: #555;
-        margin: 0 0 0.75rem;
+        color: var(--text-muted);
+        margin: 0 0 var(--space-3);
     }
+
     .description {
         white-space: pre-wrap;
+        color: var(--text);
+        margin-bottom: var(--space-3);
     }
+
     .bookmark {
-        display: inline-block;
-        margin-top: 0.5rem;
-        padding: 0.4rem 0.75rem;
-        border: 1px solid #ccc;
-        border-radius: 4px;
-        background: #fafafa;
-        color: inherit;
+        display: inline-flex;
+        align-items: center;
+        gap: var(--space-2);
+        margin-top: var(--space-2);
+        padding: 0 var(--space-3);
+        height: 36px;
+        border: 1px solid var(--border-strong);
+        border-radius: var(--radius-md);
+        background: var(--surface);
+        color: var(--text);
         text-decoration: none;
         cursor: pointer;
+        font-size: var(--font-sm);
+        font-weight: var(--weight-medium);
+        transition:
+            background var(--transition),
+            border-color var(--transition),
+            color var(--transition);
     }
-    .bookmark:focus-visible {
-        outline: 2px solid #06f;
-        outline-offset: 2px;
+
+    .bookmark:hover {
+        background: var(--surface-elevated);
+        text-decoration: none;
     }
+
     .bookmark.active {
-        background: #ffeebb;
-        border-color: #d6a800;
+        background: var(--warning-soft-bg);
+        border-color: var(--warning-border);
+        color: var(--text);
     }
+
     .chapter-list {
-        padding-left: 1.5rem;
+        padding-left: var(--space-6);
+        color: var(--text);
     }
+
+    .chapter-list li {
+        padding: var(--space-1) 0;
+    }
+
     .pages {
-        color: #777;
-        margin-left: 0.5rem;
+        color: var(--text-muted);
+        margin-left: var(--space-2);
+        font-size: var(--font-sm);
     }
 </style>