Antispam: "Mark as Spam" belongs in the Actions menu

A button pinned to every message header is a persistent control; marking spam is
a verb you reach for occasionally, like Reply or Delete. It now lives in the
Actions menu, between Labels and Delete.

That needed a slot inside the dropdown, which the catalog had no name for:
`message.actions.menu`. The host wraps it so any click closes the menu, leaving
a bundle to do nothing but act.

The UI kit gains `.wm-menu-item`, @applying the exact utilities ThreadView's own
menu items use — the same trick the sidebar shell already relies on, so a plugin
verb is indistinguishable from a core one rather than approximately similar.

Labels follow the folder as before: "Mark as Spam" in the inbox or archive,
"Mark as not spam" in the spam folder, nothing in Sent/Drafts/Trash. A message
already taught shows that, disabled, instead of offering the lesson again.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018SqJYt7jD7oUEkoY68sBZo

backend/app/plugins/types.py

@@ -26,7 +26,17 @@ JobFunc = Callable[..., Awaitable[Any]]
 
 # The frozen slot catalog (SPEC §11.3). Bundles are injected into these UI points.
 SLOT_NAMES: frozenset[str] = frozenset(
-    {"thread.toolbar", "message.actions", "message.banner", "sidebar.section", "settings.page"}
+    {
+        "thread.toolbar",
+        "message.actions",
+        # Inside the per-message Actions dropdown, for plugin verbs that belong
+        # with Reply/Print/Delete rather than sitting on the header as a button.
+        # The host closes the menu after a click, so a bundle here just acts.
+        "message.actions.menu",
+        "message.banner",
+        "sidebar.section",
+        "settings.page",
+    }
 )
 
 _NAME_RE = re.compile(r"[a-z][a-z0-9_]*")