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/register/+page.svelte

@@ -26,8 +26,8 @@
 
 <h1>Register</h1>
 <form onsubmit={submit} action="javascript:void(0)" data-testid="register-form">
-    <label>
-        Username
+    <label class="form-field">
+        <span>Username</span>
         <input
             type="text"
             bind:value={username}
@@ -38,8 +38,8 @@
             data-testid="register-username"
         />
     </label>
-    <label>
-        Password
+    <label class="form-field">
+        <span>Password</span>
         <input
             type="password"
             bind:value={password}
@@ -49,14 +49,14 @@
             data-testid="register-password"
         />
     </label>
-    <button type="submit" disabled={submitting} data-testid="register-submit">
+    <button class="primary" type="submit" disabled={submitting} data-testid="register-submit">
         {submitting ? 'Registering…' : 'Register'}
     </button>
     {#if error}
-        <p role="alert" data-testid="register-error">{error}</p>
+        <p class="form-error" role="alert" data-testid="register-error">{error}</p>
     {/if}
 </form>
-<p>
+<p class="hint">
     Already have an account? <a href="/login">Log in</a>.
 </p>
 
@@ -64,12 +64,30 @@
     form {
         display: flex;
         flex-direction: column;
-        gap: 0.75rem;
+        gap: var(--space-3);
         max-width: 24rem;
     }
-    label {
-        display: flex;
-        flex-direction: column;
-        gap: 0.25rem;
+
+    .primary {
+        background: var(--primary);
+        color: var(--primary-contrast);
+        border-color: var(--primary);
+        margin-top: var(--space-1);
+    }
+
+    .primary:hover:not(:disabled) {
+        background: var(--primary-hover);
+        border-color: var(--primary-hover);
+    }
+
+    .form-error {
+        color: var(--danger);
+        font-size: var(--font-sm);
+    }
+
+    .hint {
+        margin-top: var(--space-3);
+        color: var(--text-muted);
+        font-size: var(--font-sm);
     }
 </style>