feat: per-page carts with overview, page-scoped checkout, and split blueprints (Phase 4)

Splits the monolithic cart blueprint into three: cart_overview (GET /),
page_cart (/<page_slug>/), and cart_global (webhook, return, add).

- New page_cart.py service: get_cart_for_page(), get_calendar_entries_for_page(), get_cart_grouped_by_page()
- clear_cart_for_order() and create_order_from_cart() accept page_post_id for scoping
- Cart app hydrates page_slug via url_value_preprocessor/url_defaults/hydrate_page
- Context processor provides page-scoped cart data when g.page_post exists
- Internal API /internal/cart/summary accepts ?page_slug= for page-scoped counts
- Overview template shows page cards with item counts and totals
- Page cart template reuses show_cart() macro with page-specific header

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

bp/cart/overview_routes.py

@@ -0,0 +1,31 @@
+# bp/cart/overview_routes.py — Cart overview (list of page carts)
+
+from __future__ import annotations
+
+from quart import Blueprint, render_template, make_response
+
+from suma_browser.app.utils.htmx import is_htmx_request
+from .services import get_cart_grouped_by_page
+
+
+def register(url_prefix: str) -> Blueprint:
+    bp = Blueprint("cart_overview", __name__, url_prefix=url_prefix)
+
+    @bp.get("/")
+    async def overview():
+        from quart import g
+        page_groups = await get_cart_grouped_by_page(g.s)
+
+        if not is_htmx_request():
+            html = await render_template(
+                "_types/cart/overview/index.html",
+                page_groups=page_groups,
+            )
+        else:
+            html = await render_template(
+                "_types/cart/overview/_oob_elements.html",
+                page_groups=page_groups,
+            )
+        return await make_response(html)
+
+    return bp