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>

frontend/src/index.css

@@ -68,6 +68,49 @@
   --color-status-failed-bg: rgba(248, 81, 73, 0.15);
 }
 
+@custom-variant light (&:where([data-theme="light"], [data-theme="light"] *));
+
+/* ── Light mode overrides ─────────────────────────────────────── */
+
+html[data-theme='light'] {
+  color-scheme: light;
+
+  /* Surfaces */
+  --color-surface-0: #ffffff;
+  --color-surface-1: #f6f8fa;
+  --color-surface-2: #eef1f4;
+  --color-surface-3: #d8dee4;
+  --color-surface-4: #ced5dc;
+
+  /* Accent (darkened for text contrast on light surfaces) */
+  --color-accent-200: #245f7e;
+  --color-accent-300: #1a5068;
+  --color-accent-400: #2d6a89;
+
+  /* Text */
+  --color-text-primary: #1f2328;
+  --color-text-secondary: #656d76;
+  --color-text-tertiary: #8b949e;
+  --color-text-link: #1a5068;
+
+  /* Borders */
+  --color-border-default: rgba(0, 0, 0, 0.15);
+  --color-border-muted: rgba(0, 0, 0, 0.08);
+  --color-border-accent: rgba(57, 94, 115, 0.35);
+
+  /* Status (adjusted for light surfaces) */
+  --color-status-alive: #1a7f37;
+  --color-status-alive-bg: rgba(26, 127, 55, 0.1);
+  --color-status-dead: #cf222e;
+  --color-status-dead-bg: rgba(207, 34, 46, 0.1);
+  --color-status-active: #1a7f37;
+  --color-status-active-bg: rgba(26, 127, 55, 0.1);
+  --color-status-completed: #0969da;
+  --color-status-completed-bg: rgba(9, 105, 218, 0.1);
+  --color-status-failed: #cf222e;
+  --color-status-failed-bg: rgba(207, 34, 46, 0.1);
+}
+
 /* ── Base layer ────────────────────────────────────────────────── */
 
 @layer base {