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/lib/styles/tokens.css

@@ -0,0 +1,265 @@
+/* Design tokens — see docs/design-system.md.
+   Components consume tokens only; no raw hex or raw px in scoped styles. */
+
+:root {
+    --bg: #ffffff;
+    --surface: #f6f7f9;
+    --surface-elevated: #ffffff;
+    --border: #e3e5ea;
+    --border-strong: #c8ccd4;
+    --text: #16181d;
+    --text-muted: #5b6168;
+    --primary: #2563eb;
+    --primary-hover: #1d4ed8;
+    --primary-contrast: #ffffff;
+    --primary-soft-bg: rgb(37 99 235 / 0.08);
+    --danger: #b00020;
+    --danger-soft-bg: #fff5f5;
+    --success: #0a7d2c;
+    --warning-soft-bg: #fff5d6;
+    --warning-border: #d6a800;
+    --focus-ring: #2563eb;
+    --focus-ring-soft: rgb(37 99 235 / 0.25);
+
+    --shadow-sm: 0 1px 2px rgb(0 0 0 / 0.06);
+    --shadow-md: 0 2px 8px rgb(0 0 0 / 0.08);
+
+    --font-stack: system-ui, -apple-system, 'Segoe UI', Roboto, sans-serif;
+
+    --font-xs: 0.75rem;
+    --font-sm: 0.875rem;
+    --font-base: 1rem;
+    --font-lg: 1.125rem;
+    --font-xl: 1.5rem;
+    --font-2xl: 2rem;
+
+    --leading-tight: 1.3;
+    --leading-snug: 1.45;
+    --leading-normal: 1.55;
+
+    --weight-regular: 400;
+    --weight-medium: 500;
+    --weight-semibold: 600;
+
+    --space-1: 0.25rem;
+    --space-2: 0.5rem;
+    --space-3: 0.75rem;
+    --space-4: 1rem;
+    --space-5: 1.25rem;
+    --space-6: 1.5rem;
+    --space-8: 2rem;
+
+    --radius-sm: 4px;
+    --radius-md: 6px;
+    --radius-lg: 10px;
+    --radius-pill: 999px;
+
+    --transition: 120ms ease-out;
+
+    --icon-sm: 14px;
+    --icon-md: 18px;
+    --icon-lg: 22px;
+
+    --z-dropdown: 10;
+    --z-sticky: 50;
+    --z-modal: 100;
+    --z-toast: 1000;
+}
+
+:root[data-theme='dark'] {
+    --bg: #0f1115;
+    --surface: #161a21;
+    --surface-elevated: #1c2129;
+    --border: #2a2f37;
+    --border-strong: #3a414d;
+    --text: #e8eaed;
+    --text-muted: #9aa0a6;
+    --primary: #60a5fa;
+    --primary-hover: #3b82f6;
+    --primary-contrast: #0f1115;
+    --primary-soft-bg: rgb(96 165 250 / 0.12);
+    --danger: #f87171;
+    --danger-soft-bg: #3a1620;
+    --success: #4ade80;
+    --warning-soft-bg: #3a2e10;
+    --warning-border: #a37800;
+    --focus-ring: #60a5fa;
+    --focus-ring-soft: rgb(96 165 250 / 0.3);
+
+    --shadow-sm: 0 1px 2px rgb(0 0 0 / 0.4);
+    --shadow-md: 0 2px 8px rgb(0 0 0 / 0.5);
+}
+
+*,
+*::before,
+*::after {
+    box-sizing: border-box;
+}
+
+html,
+body {
+    margin: 0;
+    padding: 0;
+    background: var(--bg);
+    color: var(--text);
+    font-family: var(--font-stack);
+    font-size: var(--font-base);
+    line-height: var(--leading-normal);
+    -webkit-font-smoothing: antialiased;
+    -moz-osx-font-smoothing: grayscale;
+}
+
+h1,
+h2,
+h3 {
+    color: var(--text);
+    line-height: var(--leading-tight);
+    font-weight: var(--weight-semibold);
+    margin: 0 0 var(--space-3);
+}
+
+h1 {
+    font-size: var(--font-2xl);
+}
+
+h2 {
+    font-size: var(--font-xl);
+}
+
+h3 {
+    font-size: var(--font-lg);
+}
+
+p {
+    margin: 0 0 var(--space-3);
+}
+
+a {
+    color: var(--primary);
+    text-decoration: none;
+    transition: color var(--transition);
+}
+
+a:hover {
+    text-decoration: underline;
+}
+
+a:focus-visible,
+button:focus-visible,
+input:focus-visible,
+select:focus-visible,
+textarea:focus-visible,
+[role='button']:focus-visible {
+    outline: 2px solid var(--focus-ring);
+    outline-offset: 2px;
+    border-radius: var(--radius-sm);
+}
+
+button,
+input,
+select,
+textarea {
+    font: inherit;
+    color: inherit;
+}
+
+input[type='text'],
+input[type='search'],
+input[type='password'],
+input[type='number'],
+input[type='email'],
+select,
+textarea {
+    background: var(--surface);
+    color: var(--text);
+    border: 1px solid var(--border-strong);
+    border-radius: var(--radius-md);
+    padding: 0 var(--space-3);
+    height: 36px;
+    width: 100%;
+    transition:
+        border-color var(--transition),
+        background var(--transition),
+        box-shadow var(--transition);
+}
+
+textarea {
+    height: auto;
+    min-height: 6rem;
+    padding: var(--space-2) var(--space-3);
+    line-height: var(--leading-snug);
+    resize: vertical;
+}
+
+input[type='text']:focus,
+input[type='search']:focus,
+input[type='password']:focus,
+input[type='number']:focus,
+input[type='email']:focus,
+select:focus,
+textarea:focus {
+    outline: none;
+    border-color: var(--primary);
+    background: var(--surface-elevated);
+    box-shadow: 0 0 0 3px var(--focus-ring-soft);
+}
+
+input[aria-invalid='true'],
+select[aria-invalid='true'],
+textarea[aria-invalid='true'] {
+    border-color: var(--danger);
+}
+
+input:disabled,
+select:disabled,
+textarea:disabled {
+    opacity: 0.5;
+    cursor: not-allowed;
+}
+
+button {
+    cursor: pointer;
+    background: transparent;
+    border: 1px solid transparent;
+    border-radius: var(--radius-md);
+    padding: 0 var(--space-3);
+    height: 36px;
+    font-weight: var(--weight-medium);
+    color: var(--text);
+    transition:
+        background var(--transition),
+        border-color var(--transition),
+        color var(--transition);
+}
+
+button:disabled {
+    opacity: 0.5;
+    cursor: not-allowed;
+}
+
+::placeholder {
+    color: var(--text-muted);
+    opacity: 1;
+}
+
+.form-field {
+    display: flex;
+    flex-direction: column;
+    gap: var(--space-1);
+    font-size: var(--font-sm);
+    color: var(--text);
+}
+
+img {
+    max-width: 100%;
+    height: auto;
+}
+
+@media (prefers-reduced-motion: reduce) {
+    *,
+    *::before,
+    *::after {
+        transition-duration: 0ms !important;
+        animation-duration: 0ms !important;
+    }
+}