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/bookmarks/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
     import { fileUrl } from '$lib/api/client';
+    import BookImage from '@lucide/svelte/icons/book-image';
 
     let { data } = $props();
     const authenticated = $derived(data.authenticated);
@@ -14,15 +15,15 @@
 <h1>Bookmarks</h1>
 
 {#if error}
-    <p role="alert" data-testid="bookmarks-error">
+    <p class="error" role="alert" data-testid="bookmarks-error">
         Couldn't load bookmarks: {error}
     </p>
 {:else if !authenticated}
-    <p data-testid="bookmarks-signin">
+    <p class="hint" data-testid="bookmarks-signin">
         <a href="/login">Sign in</a> to see your bookmarks.
     </p>
 {:else if bookmarks.length === 0}
-    <p data-testid="bookmarks-empty">No bookmarks yet.</p>
+    <p class="hint" data-testid="bookmarks-empty">No bookmarks yet.</p>
 {:else}
     <ul class="bookmark-list" data-testid="bookmark-list">
         {#each bookmarks as b (b.id)}
@@ -36,7 +37,9 @@
                             loading="lazy"
                         />
                     {:else}
-                        <div class="cover cover-placeholder">📖</div>
+                        <div class="cover cover-placeholder">
+                            <BookImage size={22} aria-hidden="true" />
+                        </div>
                     {/if}
                 </a>
                 <div class="meta">
@@ -75,52 +78,74 @@
     .bookmark-list {
         list-style: none;
         padding: 0;
+        margin: 0;
     }
+
     .bookmark {
         display: grid;
         grid-template-columns: 64px 1fr;
-        gap: 1rem;
+        gap: var(--space-4);
         align-items: start;
-        padding: 0.75rem 0;
-        border-bottom: 1px solid #eee;
+        padding: var(--space-3) 0;
+        border-bottom: 1px solid var(--border);
     }
+
     .cover-link {
         display: block;
         line-height: 0;
     }
+
     .cover {
         width: 64px;
         height: 96px;
         object-fit: cover;
-        border-radius: 4px;
-        background: #f0f0f0;
+        border-radius: var(--radius-md);
+        background: var(--surface);
     }
+
     .cover-placeholder {
         display: flex;
         align-items: center;
         justify-content: center;
-        font-size: 1.5rem;
-        color: #999;
+        color: var(--text-muted);
         user-select: none;
     }
+
     .meta {
         display: flex;
         flex-direction: column;
-        gap: 0.15rem;
+        gap: var(--space-1);
         min-width: 0;
     }
+
     .title {
-        font-weight: 600;
-        font-size: 1rem;
+        font-weight: var(--weight-semibold);
+        font-size: var(--font-base);
+        color: var(--text);
     }
+
+    .title:hover {
+        color: var(--primary);
+    }
+
     .target {
-        font-size: 0.95rem;
+        font-size: var(--font-sm);
     }
+
     .muted {
-        color: #888;
+        color: var(--text-muted);
     }
+
     .created {
-        color: #888;
-        font-size: 0.85rem;
+        color: var(--text-muted);
+        font-size: var(--font-xs);
+    }
+
+    .error {
+        color: var(--danger);
+    }
+
+    .hint {
+        color: var(--text-muted);
     }
 </style>