Autocomplete existing tags in the upload and manage forms

A small shared widget (static/tagcomplete.js) completes the tag after the last
comma from the existing tag list, keeping earlier tags intact. It is wired to
every comma-separated tag input: the upload form, and the manage page's per-row
edit field and bulk add/remove fields.

Suggestions come from /tags.json, which the generator already writes on every
build and nginx serves statically, so no server change is needed; the list is
fetched once, cached, and invalidated after an edit that may add a tag. Keyboard
navigation (up/down, enter/tab, escape) and click selection are supported.

templates/upload.html

@@ -163,13 +163,14 @@
 
     <div class="field">
       <label for="tags">Tags</label>
-      <input type="text" id="tags" placeholder="comma-separated, lowercase" autocomplete="off">
+      <input type="text" id="tags" placeholder="comma-separated, lowercase" autocomplete="off" data-tagcomplete>
     </div>
 
     <button class="submit-btn" id="submit-btn" disabled>Upload photo</button>
     <div id="feedback"></div>
   </div>
 
+  <script src="/static/tagcomplete.js"></script>
   <script>
     const fileInput = document.getElementById('file-input');
     const preview   = document.getElementById('preview');