fix: use crypto.getRandomValues for randId to eliminate collision risk (#1282)

* fix(randId): use crypto.getRandomValues for connection IDs

Math.random() gives ~30 bits of entropy and no cryptographic guarantees.
Switch to crypto.getRandomValues over a Uint32 buffer so device / saved-
connection IDs get full 32-bit range and don't collide across concurrent
transports.

Closes #1273

* fix: assert Uint32Array index for strict TS7 compat

Under strict + noUncheckedIndexedAccess (TS7 default), buf[0] is
number | undefined. Uint32Array(1) guarantees length 1, so assert
non-null to satisfy the number return type.

apps/web/src/core/utils/randId.test.ts

@@ -1,44 +1,25 @@
-import { beforeEach, describe, expect, it, vi } from "vitest";
+import { describe, expect, it } from "vitest";
 import { randId } from "./randId.ts";
 
 describe("randId", () => {
-  beforeEach(() => {
-    vi.restoreAllMocks();
-  });
-
-  it("returns a number", () => {
-    const result = randId();
-    expect(typeof result).toBe("number");
-  });
-
   it("returns an integer", () => {
     const result = randId();
+    expect(typeof result).toBe("number");
     expect(Number.isInteger(result)).toBe(true);
   });
 
-  it("uses Math.random to generate the number", () => {
-    const mockRandom = vi.spyOn(Math, "random").mockReturnValue(0.5);
-    const result = randId();
-
-    expect(mockRandom).toHaveBeenCalled();
-    expect(result).toBe(Math.floor(0.5 * 1e9));
-  });
-
-  it("returns a value between 0 and 1e9 (exclusive)", () => {
+  it("returns a value within the uint32 range", () => {
     const result = randId();
     expect(result).toBeGreaterThanOrEqual(0);
-    expect(result).toBeLessThan(1e9);
+    expect(result).toBeLessThanOrEqual(0xffffffff);
   });
 
-  it("returns different values on subsequent calls", () => {
-    vi.spyOn(Math, "random").mockRestore();
-
-    const results = new Set();
-
-    for (let i = 0; i < 100; i++) {
+  it("returns unique values across many calls", () => {
+    const results = new Set<number>();
+    for (let i = 0; i < 1000; i++) {
       results.add(randId());
     }
-
-    expect(results.size).toBeGreaterThan(95);
+    // With 32 bits of entropy over 1000 samples, collisions are vanishingly rare.
+    expect(results.size).toBe(1000);
   });
 });

apps/web/src/core/utils/randId.ts

@@ -1,3 +1,5 @@
-export const randId = () => {
-  return Math.floor(Math.random() * 1e9);
+export const randId = (): number => {
+  const buf = new Uint32Array(1);
+  crypto.getRandomValues(buf);
+  return buf[0]!;
 };