Add dark/light mode toggle with adaptive badge colors

Implement theme switching via sun/moon toggle in nav bar. Dark
remains the default; light mode overrides surface, text, border,
accent, and status color tokens. Preference persists in localStorage
and falls back to prefers-color-scheme. An inline script in
index.html prevents flash of wrong theme on load.

Define a Tailwind v4 @custom-variant for light mode and update all
badge components (encounter method, rule, condition) to use
light:bg-{color}-100 / light:text-{color}-700 for readable contrast
on light surfaces.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.beans/nuzlocke-tracker-dpw7--modernize-website-design-and-look-and-feel.md

@@ -91,7 +91,7 @@ Self-host **Geist** (or Inter/JetBrains Mono pairing):
 - [x] Update all page-level backgrounds and containers
 - [x] Update modal styles (EncounterModal, StatusChangeModal, etc.)
 - [x] Update badge/indicator styles (TypeBadge, RuleBadges, EncounterMethodBadge)
-- [ ] Add dark/light mode toggle to nav
+- [x] Add dark/light mode toggle to nav
 - [x] Polish hover states and transitions across all interactive elements
 - [ ] Add automated Playwright accessibility and mobile layout tests
 - [ ] Verify accessibility (contrast ratios, focus indicators)