Add a website widget, so a page nobody modelled as a message can hang on a wall

A tile that draws whatever an address serves: no binding, no flow, just an
iframe. Only http(s) loads — a `javascript:` src would run in the app's own
origin, and a dashboard is a document several people can edit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KocbsBHWme1kfCHVhrgBnw

backend/fluksio/flow/dashboards.py

@@ -65,6 +65,7 @@ WidgetType = Literal[
     "clock",
     "media",
     "player",
+    "embed",
     # Input
     "button",
     "switch",
@@ -113,8 +114,8 @@ WIDGET_DTYPES: dict[str, set[str]] = {
     # duration in one reading, because they are one thing and a player drawn
     # from five separate messages would redraw itself five times.
     "player": {"record"},
-    # An icon maps weather strings, bool hints and numbers alike, and a clock
-    # binds nothing at all, so neither has a row to be held to.
+    # An icon maps weather strings, bool hints and numbers alike; a clock reads
+    # the wall and an embed a page of its own, so none has a row to be held to.
 }