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/__tests__/combatant-row.test.tsx

@@ -75,7 +75,7 @@ describe("CombatantRow", () => {
 	it("active combatant gets active border styling", () => {
 		const { container } = renderRow({ isActive: true });
 		const row = container.firstElementChild;
-		expect(row?.className).toContain("border-accent/40");
+		expect(row?.className).toContain("border-active-row-border");
 	});
 
 	it("unconscious combatant (currentHp === 0) gets dimmed styling", () => {