feat(ui): default theme to system (follow OS light/dark)

Light has been signed off, so the theme store's default choice is now
`system` (it was `dark` during the incremental migration). This matches
the app.html pre-paint guard, which already resolved an unset choice via
prefers-color-scheme — removing the brief boot-time mismatch where the
store re-pinned dark. Users still pin light/dark via the account-menu
picker. Updates the store default + its test and the design-system /
finalize-plan docs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/frontend/tests/theme.test.ts

@@ -42,9 +42,10 @@ describe("theme store", () => {
 		vi.restoreAllMocks();
 	});
 
-	it("defaults to dark and applies it to the document", async () => {
+	it("defaults to system and applies the resolved theme", async () => {
 		const { theme } = await freshStore();
-		expect(theme.choice).toBe("dark");
+		expect(theme.choice).toBe("system");
+		// freshStore() leaves the OS at dark (prefersLight = false).
 		expect(theme.resolved).toBe("dark");
 		expect(document.documentElement.dataset.theme).toBe("dark");
 	});
@@ -80,9 +81,9 @@ describe("theme store", () => {
 		expect(document.documentElement.dataset.theme).toBe("light");
 	});
 
-	it("falls back to dark for an unrecognised stored value", async () => {
+	it("falls back to system for an unrecognised stored value", async () => {
 		localStorage.setItem(STORAGE_KEY, "neon");
 		const { theme } = await freshStore();
-		expect(theme.choice).toBe("dark");
+		expect(theme.choice).toBe("system");
 	});
 });