Add simple manifest.json (closes #262)

src/library/entry.cr

@@ -108,7 +108,7 @@ class Entry
   end
 
   def cover_url
-    return "#{Config.current.base_url}img/icon.png" if @err_msg
+    return "#{Config.current.base_url}img/icons/icon.png" if @err_msg
 
     unless @book.entry_cover_url_cache
       TitleInfo.new @book.dir do |info|

src/library/title.cr

@@ -411,7 +411,7 @@ class Title
     cached_cover_url = @cached_cover_url
     return cached_cover_url unless cached_cover_url.nil?
 
-    url = "#{Config.current.base_url}img/icon.png"
+    url = "#{Config.current.base_url}img/icons/icon.png"
     readable_entries = @entries.select &.err_msg.nil?
     if readable_entries.size > 0
       url = readable_entries[0].cover_url

src/views/components/head.html.ecr

@@ -6,6 +6,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <link rel="stylesheet" href="<%= base_url %>css/mango.css" />
   <link rel="icon" href="<%= base_url %>favicon.ico">
+  <link rel="manifest" href="<%= base_url %>manifest.json">
 
   <script src="https://polyfill.io/v3/polyfill.min.js?features=MutationObserver%2Cdefault%2CmatchMedia&flats=gated"></script>
   <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>

src/views/layout.html.ecr

@@ -36,7 +36,7 @@
           <div class="uk-navbar-toggle" uk-navbar-toggle-icon="uk-navbar-toggle-icon" uk-toggle="target: #mobile-nav"></div>
         </div>
         <div class="uk-navbar-left uk-visible@m">
-          <a class="uk-navbar-item uk-logo" href="<%= base_url %>"><img src="<%= base_url %>img/icon.png" style="width:90px;height:90px;"></a>
+          <a class="uk-navbar-item uk-logo" href="<%= base_url %>"><img src="<%= base_url %>img/icons/icon.png" style="width:90px;height:90px;"></a>
           <ul class="uk-navbar-nav">
             <li><a href="<%= base_url %>">Home</a></li>
             <li><a href="<%= base_url %>library">Library</a></li>