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/entries.ts

@@ -63,6 +63,7 @@ export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
             content="width=device-width, initial-scale=1.0"
           />
           <title>${emailData.subject}</title>
+          <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
           <style>
             body {
               font-family: sans-serif;