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/tests/plugins/test_antispam_api.py

@@ -181,7 +181,7 @@ async def test_stats_include_engine_state_when_it_is_up(ctx: _Ctx) -> None:
 
 
 async def test_ui_bundles_are_served_and_gated(ctx: _Ctx) -> None:
-    for name in ("settings.js", "banner.js", "actions.js"):
+    for name in ("settings.js", "banner.js", "menu.js"):
         r = await ctx.client_a.get(f"{BASE}/ui/{name}")
         assert r.status_code == 200
         assert "javascript" in r.headers["content-type"]