#267: jump to top/bottom buttons for long pastes and the editor

Fixed-position Top/Bottom pills appear only when content exceeds 2x
viewport height (window scroll on paste view, textarea scroll on /new).
New static/jump.js drives them; markup added to paste.html and new.html.

internal/web/templates/new.html

@@ -33,6 +33,10 @@
       <div class="spacer spacer-flex"></div>
       <button class="btn" id="create">Create</button>
     </div>
+    <div class="jumpnav hidden" id="jumpnav" data-jump-scroll="#content">
+      <button type="button" class="btn jump-btn" data-jump="top">Top</button>
+      <button type="button" class="btn jump-btn" data-jump="bottom">Bottom</button>
+    </div>
   </div>
 
   <div class="pane-r">
@@ -90,4 +94,5 @@
   </div>
 </div>
 <script src="/static/new.js" defer></script>
+<script src="/static/jump.js" defer></script>
 {{template "foot" .}}