feat(generator): add -default-view to choose timeline or grid as site root

The chosen view is published at /, the other under /timeline/ or
/grid/. Page paths, pager links and the cross-links between the views
are derived from the setting. The directory named after the default
view is removed on build, since it can only be a leftover from before
a switch.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

templates/day.html

@@ -23,7 +23,7 @@
         <p class="handle">{{.Handle}}</p>
         <p class="pcount"><strong>{{.Total}}</strong> posts</p>
         <p class="pbio">{{.Bio}}{{if .BioAlt}}<br><span class="ml">{{.BioAlt}}</span>{{end}}</p>
-        <a href="/" class="rss-a">← Timeline</a>
+        <a href="{{.TimelineURL}}" class="rss-a">← Timeline</a>
       </div>
     </div>
   </header>