Add dark and light theme with OS preference support

Follow OS color scheme by default, with a three-way toggle
(System / Light / Dark) in the kebab menu. Light theme uses warm,
neutral tones with soft card-to-background contrast. Semantic colors
(damage, healing, conditions) keep their hue across themes.

Closes #10

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/components/ui/overflow-menu.tsx

@@ -7,6 +7,7 @@ export interface OverflowMenuItem {
 	readonly label: string;
 	readonly onClick: () => void;
 	readonly disabled?: boolean;
+	readonly keepOpen?: boolean;
 }
 
 interface OverflowMenuProps {
@@ -58,7 +59,7 @@ export function OverflowMenu({ items }: OverflowMenuProps) {
 							disabled={item.disabled}
 							onClick={() => {
 								item.onClick();
-								setOpen(false);
+								if (!item.keepOpen) setOpen(false);
 							}}
 						>
 							{item.icon}