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/ingestion/image.go

@@ -24,6 +24,13 @@ func NormalizeImageURL(raw string) string {
 	if q.Get("width") == "" {
 		return raw
 	}
+	// The Guardian signs each (width, quality, fit, …) combination with `s=`.
+	// Changing width without re-signing yields a 401 "invalid signature",
+	// so leave signed URLs alone — the parser is now responsible for picking
+	// the widest media:content variant up front.
+	if q.Get("s") != "" {
+		return raw
+	}
 	q.Set("width", "1200")
 	u.RawQuery = q.Encode()
 	return u.String()