Add dark mode based on device color-scheme preference

Move the color palette into CSS custom properties (:root in
globals.scss) with a prefers-color-scheme: dark override, and point the
existing $color-* Sass variables at them so every component picks up
dark mode automatically. Replaced the handful of rgba($color, alpha)
call sites with color-mix(), since Sass can't alpha-blend a color that's
now a CSS variable at compile time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/styles/_variables.scss

@@ -1,12 +1,16 @@
-$color-text: #1a1a1a;
-$color-text-muted: #6b6b6b;
-$color-background: #ffffff;
-$color-surface: #f7f7f5;
-$color-border: #e2e0dc;
-$color-primary: #b5432b;
-$color-primary-contrast: #ffffff;
-$color-danger: #a33;
-$color-success: #2f7a4f;
+// These resolve to the CSS custom properties defined in globals.scss, which
+// swap values under `prefers-color-scheme: dark` — keep referencing them by
+// these names rather than hardcoding hex so components pick up dark mode
+// automatically.
+$color-text: var(--color-text);
+$color-text-muted: var(--color-text-muted);
+$color-background: var(--color-background);
+$color-surface: var(--color-surface);
+$color-border: var(--color-border);
+$color-primary: var(--color-primary);
+$color-primary-contrast: var(--color-primary-contrast);
+$color-danger: var(--color-danger);
+$color-success: var(--color-success);
 
 $space-1: 0.25rem;
 $space-2: 0.5rem;

src/styles/globals.scss

@@ -1,5 +1,33 @@
 @use './variables' as *;
 
+:root {
+  --color-text: #1a1a1a;
+  --color-text-muted: #6b6b6b;
+  --color-background: #ffffff;
+  --color-surface: #f7f7f5;
+  --color-border: #e2e0dc;
+  --color-primary: #b5432b;
+  --color-primary-contrast: #ffffff;
+  --color-danger: #a33333;
+  --color-success: #2f7a4f;
+  color-scheme: light;
+}
+
+@media (prefers-color-scheme: dark) {
+  :root {
+    --color-text: #f2ede6;
+    --color-text-muted: #a89e8f;
+    --color-background: #16130f;
+    --color-surface: #201c17;
+    --color-border: #3a342a;
+    --color-primary: #d9694a;
+    --color-primary-contrast: #ffffff;
+    --color-danger: #ef6b62;
+    --color-success: #4fbb7c;
+    color-scheme: dark;
+  }
+}
+
 *,
 *::before,
 *::after {