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.

static/style.css

@@ -476,3 +476,26 @@ body {
   font-family: var(--serif);
   margin-top: 16px;
 }
+
+/* Tag autocomplete dropdown (tagcomplete.js), shared by upload and manage. */
+.tagac-menu {
+  position: absolute;
+  z-index: 50;
+  background: var(--bg1);
+  border: 1px solid var(--bg2);
+  max-height: 220px;
+  overflow-y: auto;
+  box-shadow: 0 6px 18px rgba(0, 0, 0, 0.4);
+}
+.tagac-item {
+  padding: 7px 11px;
+  font-family: var(--serif);
+  font-size: 14px;
+  color: var(--fg2);
+  cursor: pointer;
+}
+.tagac-item.sel,
+.tagac-item:hover {
+  background: var(--bg2);
+  color: var(--fg);
+}