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>

backend/Cargo.lock

@@ -1033,7 +1033,7 @@ checksum = "5e5032e24019045c762d3c0f28f5b6b8bbf38563a65908389bf7978758920897"
 
 [[package]]
 name = "mangalord"
-version = "0.11.0"
+version = "0.13.0"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.12.0"
+version = "0.13.0"
 edition = "2021"
 
 [lib]