Namespace quest definition icon keys with legacy back-compat

Add core/icons.py as the one place icon keys are validated and
normalized. A definition icon is now stored as lucide:<name>,
fa:<name> or emoji:<payload> (at most 8 code points, no whitespace,
control characters or markup). Create and edit validate strictly and
store the canonical form; a legacy bare name is stored as
lucide:<name>, and None still clears the icon.

Reads normalize leniently so existing bare-name rows need no data
migration: the admin quest-definition response and the panel snapshot's
per-instance icon both return the namespaced form.

Task 707994b7.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/routes_admin.py

@@ -336,6 +336,7 @@ from api.nestquest_core import (
     core_dao_children,
     core_dao_rules,
     core_history,
+    core_icons,
     core_materialize,
     core_presence_management,
     core_quest_definitions,
@@ -1205,7 +1206,7 @@ def _quest_definition_response(
         id=bundle.definition.id,
         title=bundle.definition.title,
         description=bundle.definition.description,
-        icon=bundle.definition.icon,
+        icon=core_icons.normalize_icon_for_read(bundle.definition.icon),
         is_active=bundle.definition.is_active,
         skip_on_away=bundle.definition.skip_on_away,
         rule=_rule_response(bundle.rule),