Remove widget system — fully replaced by fragment composition

Delete shared/contracts/widgets.py, shared/services/widget_registry.py,
and shared/services/widgets/ (empty stubs). Remove register_all_widgets()
from factory and widgets Jinja global from jinja_setup. Zero consumers
remain — all cross-app UI composition now uses the fragment API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/contracts/widgets.py

@@ -1,49 +0,0 @@
-"""Widget descriptors for cross-domain UI composition.
-
-Each widget type describes a UI fragment that one domain contributes to
-another domain's page.  Host apps iterate widgets generically — they never
-name the contributing domain.
-"""
-from __future__ import annotations
-
-from dataclasses import dataclass
-from typing import Callable
-
-
-@dataclass(frozen=True, slots=True)
-class NavWidget:
-    """Renders nav items on a container page (entries, calendars, markets)."""
-    domain: str
-    order: int
-    context_fn: Callable   # async (session, *, container_type, container_id, **kw) -> dict
-    template: str
-
-
-@dataclass(frozen=True, slots=True)
-class CardWidget:
-    """Decorates content cards in listings with domain data."""
-    domain: str
-    order: int
-    batch_fn: Callable     # async (session, post_ids) -> dict[int, list]
-    context_key: str       # key injected into each post dict
-    template: str
-
-
-@dataclass(frozen=True, slots=True)
-class AccountPageWidget:
-    """Sub-page under /auth/<slug>/."""
-    domain: str
-    slug: str
-    label: str
-    order: int
-    context_fn: Callable   # async (session, *, user_id, **kw) -> dict
-    template: str
-
-
-@dataclass(frozen=True, slots=True)
-class AccountNavLink:
-    """Nav link on account page (internal or external)."""
-    label: str
-    order: int
-    href_fn: Callable      # () -> str
-    external: bool = False