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.ts

@@ -0,0 +1,23 @@
+import { Context } from "hono";
+import { Env } from "../types";
+
+export const FAVICON_PATH = "/favicon.svg";
+
+// Project favicon — reuses the header's envelope logo (brand orange #f6821f),
+// rendered as a white envelope on a rounded orange square for legibility at 16px.
+export const FAVICON_SVG = `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32" width="32" height="32">
+  <rect width="32" height="32" rx="7" fill="#f6821f"/>
+  <g fill="none" stroke="#ffffff" stroke-width="2.2" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M7 9h18c1.1 0 2 .9 2 2v10c0 1.1-.9 2-2 2H7c-1.1 0-2-.9-2-2V11c0-1.1.9-2 2-2z"/>
+    <polyline points="27,11 16,18.5 5,11"/>
+  </g>
+</svg>`;
+
+export function handle(_c: Context<{ Bindings: Env }>): Response {
+  return new Response(FAVICON_SVG, {
+    headers: {
+      "Content-Type": "image/svg+xml; charset=utf-8",
+      "Cache-Control": "public, max-age=86400",
+    },
+  });
+}