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>

tests/test_api_admin_questdef_list.py

@@ -135,7 +135,7 @@ async def test_admin_lists_active_and_inactive_definitions_with_full_shape(
                 "id": active_id,
                 "title": "Tidy the den",
                 "description": "Put everything back where it belongs.",
-                "icon": "toy-box",
+                "icon": "lucide:toy-box",
                 "is_active": True,
                 "skip_on_away": True,
                 "rule": {