Switch encoder default back to mode 4C?

Thinking through the upgrade plan, I think we can make the transition
from 4C to B fairly painless -- if the decoder has auto-detect logic. So
in line with that hope, we'll leave 4C as the default mode in 0.6.0.

Author: sz3

web/index.html

@@ -271,7 +271,7 @@
 
 <input style="display:none;" type="file" name="file_input" id="file_input" onchange="Main.fileInput(this);" />
 
-<div id="nav-container" class="mode-b">
+<div id="nav-container" class="mode-4c">
     <div class="bg" onclick="Main.blurNav();"></div>
     <button id="nav-button" tabindex="0" onclick="Main.clickNav();">
       <span class="icon-bar"></span>