fix(ssr): clear the budget timer once a raced read answers

The mood and dashboard prefetches and the sign-in location lookup raced
their work against a bare setTimeout that was never cleared, leaving
one live timer behind per render or sign-in after the work had already
answered. A shared helper races the work and clears the timer.

src/lib/async/__tests__/settle-within.test.ts

@@ -0,0 +1,34 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { settleWithin } from "../settle-within";
+
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe("settleWithin", () => {
+  it("answers with the work when it lands inside the budget, and leaves no timer", async () => {
+    vi.useFakeTimers();
+    await expect(
+      settleWithin(Promise.resolve("done"), 1_500, null),
+    ).resolves.toBe("done");
+    // A bare race left the budget timer pending after the work answered.
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it("answers with the fallback once the budget runs out", async () => {
+    vi.useFakeTimers();
+    const pending = settleWithin(new Promise<string>(() => {}), 1_500, null);
+    await vi.advanceTimersByTimeAsync(1_500);
+    await expect(pending).resolves.toBeNull();
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it("passes a rejection inside the budget through, and clears the timer", async () => {
+    vi.useFakeTimers();
+    await expect(
+      settleWithin(Promise.reject(new Error("boom")), 1_500, null),
+    ).rejects.toThrow("boom");
+    expect(vi.getTimerCount()).toBe(0);
+  });
+});