Create categories and tags in place from every assignment picker

Category and tag inputs across the transaction editor, Analyse
corrections, and merchant defaults now mint missing entries without a
detour through the registry pages. A bare name lands under the kind's
root, "Parent / Name" targets that parent, and typing an existing name
selects it instead of duplicating. Enter only creates when nothing
matches, server rejections surface inline in the dropdown, and
assignment pickers offer leaf categories only — the shape the server
validates.

Mutations now return the accepted state so callers can select the id
the server just minted, and the revision-keyed remounts on Transactions
and the registry pages are gone: they closed the open modal and threw
away pending edits the moment any in-modal creation committed.

web/src/styles.css

@@ -966,6 +966,23 @@ tbody tr:hover {
   color: #546779;
   padding: 0 5px;
 }
+/* Inline tag creation inside the picker: a small input plus one button, so a
+   missing tag never forces a detour through the Tags page. */
+.tag-add {
+  display: inline-flex;
+  align-items: center;
+  gap: 5px;
+}
+.tag-add input {
+  width: 140px;
+  padding: 6px 9px;
+  font-size: 12px;
+}
+.tag-add-error {
+  flex-basis: 100%;
+  color: var(--danger);
+  font-size: 12px;
+}
 .check-chip {
   display: inline-flex;
   align-items: center;
@@ -2075,6 +2092,17 @@ footer span:first-child {
   color: var(--muted);
   font-size: 12px;
 }
+.combo-option.create {
+  color: var(--emerald);
+  font-weight: 600;
+}
+.combo-option.create svg {
+  width: 14px;
+  height: 14px;
+}
+.combo-empty.error {
+  color: var(--danger);
+}
 /* The proposed side of a review row is editable in place: compact combobox
    inputs so a correction fits the diff card, removable chips for tags. */
 .diff-value .combo > input {