feat: add tags/labels for saved items

- Add free-form tags per item: chip display with remove, inline add (comma-separated, grid view only), and a toolbar tag-filter popover listing every tag in use
- Search now also matches tags; tags sync across devices like every other item field
- Fix Firefox manifest.json compliance for AMO submission: replace the placeholder gecko id with a real UUID, add the now-required data_collection_permissions (bookmarksInfo, since synced items are personal browsing/bookmark data), and bump strict_min_version to 140.0 (142.0 for gecko_android) to match
- Add description.md (elevator pitch / short / detailed descriptions for store listings), PRIVACY.md, and .gitignore for the dist/ packaging output
- Reword all em-dash punctuation across docs and code comments per user preference

newtab.css

@@ -259,6 +259,103 @@ select#sortSelect {
   line-height: 1.4;
 }
 
+/* --- Tags filter popover --- */
+.tags-wrap {
+  position: relative;
+  display: inline-block;
+}
+.tags-panel {
+  display: block;
+  position: absolute;
+  top: calc(100% + 6px);
+  right: 0;
+  left: auto;
+  z-index: 20;
+  width: 220px;
+  max-height: 260px;
+  overflow-y: auto;
+  background: var(--surface);
+  border: 1px solid var(--hairline);
+  border-radius: 10px;
+  box-shadow: var(--shadow);
+  padding: 14px;
+}
+.tags-panel[hidden] {
+  display: none;
+}
+.tags-list {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 6px;
+}
+
+/* --- Tag chips (shared by cards and the tags-filter popover) --- */
+.tag-chip {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  font-size: 11px;
+  padding: 3px 8px;
+  border-radius: 999px;
+  background: var(--accent-soft);
+  color: var(--clay-deep);
+  border: none;
+  font-family: inherit;
+  cursor: pointer;
+}
+.tag-chip-label {
+  cursor: pointer;
+}
+.tag-chip:hover .tag-chip-label,
+.tag-chip-filter:hover {
+  text-decoration: underline;
+}
+.tag-remove {
+  background: none;
+  border: none;
+  color: var(--clay-deep);
+  cursor: pointer;
+  font-size: 12px;
+  line-height: 1;
+  padding: 0;
+  opacity: 0.7;
+}
+.tag-remove:hover {
+  opacity: 1;
+}
+
+.card-tags {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 6px;
+  margin-top: 2px;
+}
+.tag-add-btn {
+  font-size: 11px;
+  padding: 3px 8px;
+  border-radius: 999px;
+  background: transparent;
+  border: 1px dashed var(--hairline-strong);
+  color: var(--muted);
+  cursor: pointer;
+  font-family: inherit;
+}
+.tag-add-btn:hover {
+  color: var(--ink);
+  border-color: var(--clay);
+}
+.tag-input {
+  font-size: 11px;
+  padding: 3px 8px;
+  border-radius: 999px;
+  border: 1px solid var(--hairline);
+  background: var(--paper);
+  color: var(--ink);
+  font-family: inherit;
+  width: 120px;
+}
+
 /* --- Shelf: grid view (library index cards) --- */
 .shelf[data-view="grid"] {
   display: grid;