feat(favicon): serve project favicon reusing the header envelope logo

Serve an inline SVG icon at /favicon.svg and /favicon.ico and link it
from the shared Layout and the standalone entry view, so the admin UI,
status page, and entry pages stop emitting /favicon.ico 404s. Doubles
as the fallback for the upcoming per-feed favicon feature.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/routes/favicon.test.ts

@@ -0,0 +1,29 @@
+import { describe, it, expect } from "vitest";
+import worker from "../index";
+import { createMockEnv } from "../test/setup";
+import type { Env } from "../types";
+
+function req(path: string): Request {
+  return new Request(`https://test.getmynews.app${path}`);
+}
+
+describe("project favicon", () => {
+  it("serves an SVG favicon at /favicon.svg", async () => {
+    const env = createMockEnv() as unknown as Env;
+    const res = await worker.fetch(req("/favicon.svg"), env);
+    expect(res.status).toBe(200);
+    expect(res.headers.get("Content-Type")).toMatch(/^image\/svg\+xml/);
+    expect(res.headers.get("Cache-Control")).toContain("max-age");
+    const body = await res.text();
+    expect(body).toContain("<svg");
+  });
+
+  it("serves the same icon at /favicon.ico", async () => {
+    const env = createMockEnv() as unknown as Env;
+    const res = await worker.fetch(req("/favicon.ico"), env);
+    expect(res.status).toBe(200);
+    expect(res.headers.get("Content-Type")).toMatch(/^image\/svg\+xml/);
+    const body = await res.text();
+    expect(body).toContain("<svg");
+  });
+});