Local mode, Makefile, image fixes, Pete avatar

- Add -local flag: web/RSS-only mode that skips Matrix login and posting,
  so the web UI can be exercised against live feeds without credentials
- Add Makefile (build/local/seed/test/clean) that handles Tailwind +
  go build in one shot
- Fix Guardian thumbnails: NormalizeImageURL was rewriting width=1200
  onto signed i.guim.co.uk URLs, invalidating the s= signature and
  returning 401. Leave signed URLs alone and pick the widest
  media:content variant up front instead
- Use pete.avif as the header logo, favicon, and footer mark; drop the
  unused leaf.svg

internal/web/templates/layout.html

@@ -8,7 +8,7 @@
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
   <link href="https://fonts.googleapis.com/css2?family=Fredoka:wght@400;500;600;700&family=Nunito:wght@400;600;700&display=swap" rel="stylesheet">
   <link rel="stylesheet" href="/static/css/output.css">
-  <link rel="icon" href="/static/img/leaf.svg" type="image/svg+xml">
+  <link rel="icon" href="/static/img/pete.avif" type="image/avif">
   <script>
     // Pick a palette phase from the browser clock and update it each minute.
     (function () {
@@ -32,7 +32,8 @@
   <header class="mx-auto max-w-6xl px-4 pt-8 pb-4 sm:pt-12">
     <div class="flex items-center justify-between gap-4">
       <a href="/" class="flex items-center gap-3 group">
-        <span class="inline-flex h-12 w-12 items-center justify-center rounded-2xl bg-[color:var(--card)] shadow-pete border-2 border-[color:var(--ink)]/10 text-2xl group-hover:-rotate-6 transition-transform">🌿</span>
+        <img src="/static/img/pete.avif" alt="Pete" width="48" height="48"
+             class="h-12 w-12 rounded-2xl bg-[color:var(--card)] shadow-pete border-2 border-[color:var(--ink)]/10 object-cover group-hover:-rotate-6 transition-transform">
         <span class="font-display text-3xl font-bold tracking-tight">{{.SiteTitle}}</span>
         <span class="hidden sm:inline rounded-full bg-[color:var(--accent)]/20 px-3 py-1 text-xs font-semibold uppercase tracking-wider text-[color:var(--ink)]/70" data-phase-label>day</span>
       </a>
@@ -54,6 +55,8 @@
   </main>
 
   <footer class="mx-auto max-w-6xl px-4 pb-10 text-center text-sm text-[color:var(--ink)]/50">
+    <img src="/static/img/pete.avif" alt="" width="20" height="20"
+         class="inline-block h-5 w-5 rounded-full object-cover align-[-4px] mr-1">
     served by <span class="font-semibold">Pete</span> · also a Matrix bot · <span data-clock>—</span>
   </footer>