info_panel: suppress flat-tag flash when category fetch is pending

When a category fetch is about to fire (Rule34/Safebooru.org/
Moebooru on first click), the info panel was rendering the full
flat tag list, then ~200ms later re-rendering with categorized
tags. The re-layout from flat→categorized looked like a visual
hitch.

Fix: new _categories_pending flag on InfoPanel. When True, the
flat-tag fallback branch is skipped — the tags area stays empty
until categories arrive and render in one clean pass.

  _ensure_post_categories_async sets _categories_pending = True
    before scheduling the fetch (or False if no fetcher = Danbooru)
  _on_categories_updated clears _categories_pending = False

Visual result:
  Danbooru/e621:        instant (inline, no flag)
  Gelbooru with auth:   instant (background prefetch beat the click)
  Rule34/SB.org/Moebooru: empty ~200ms → categories appear cleanly
                          (no flat→categorized re-layout)

booru_viewer/gui/info_panel.py

@@ -79,6 +79,7 @@ class InfoPanel(QWidget):
 
     def __init__(self, parent: QWidget | None = None) -> None:
         super().__init__(parent)
+        self._categories_pending = False
         layout = QVBoxLayout(self)
         layout.setContentsMargins(6, 6, 6, 6)
 
@@ -164,8 +165,11 @@ class InfoPanel(QWidget):
                     btn.setStyleSheet(style)
                     btn.clicked.connect(lambda checked, t=tag: self.tag_clicked.emit(t))
                     self._tags_flow.addWidget(btn)
-        else:
-            # Fallback: flat tag list (Gelbooru, Moebooru)
+        elif not self._categories_pending:
+            # Flat tag fallback — only when no category fetch is
+            # in-flight. When a fetch IS pending, leaving the tags
+            # area empty avoids the flat→categorized re-layout hitch
+            # (categories arrive ~200ms later and render in one pass).
             for tag in post.tag_list[:100]:
                 btn = QPushButton(tag)
                 btn.setFlat(True)

booru_viewer/gui/main_window.py

@@ -166,14 +166,15 @@ class BooruApp(QMainWindow):
         No-op if the active client doesn't have a CategoryFetcher
         (Danbooru/e621 categorize inline, no fetcher needed).
 
-        Does NOT check post.tag_categories — partial cache composes
-        from the background prefetch can leave the post at e.g.
-        5/40 coverage. ensure_categories checks 100% cache coverage
-        internally and fetches the remainder if needed.
+        Sets _categories_pending on the info panel so it skips the
+        flat-tag fallback render (avoids the flat→categorized
+        re-layout hitch). The flag clears when categories arrive.
         """
         client = self._make_client()
         if client is None or client.category_fetcher is None:
+            self._info_panel._categories_pending = False
             return
+        self._info_panel._categories_pending = True
         fetcher = client.category_fetcher
         signals = self._signals
 
@@ -195,6 +196,7 @@ class BooruApp(QMainWindow):
         place by the CategoryFetcher, so we just call the panel's
         set_post / set_post_tags again to pick up the new dict.
         """
+        self._info_panel._categories_pending = False
         if not post or not post.tag_categories:
             return
         idx = self._grid.selected_index