Add register_sx_layout infrastructure, convert account/federation/orders

Phase 0: Add _ctx_to_env() and render_to_sx_with_env() to shared/sx/helpers.py,
register_sx_layout() to shared/sx/layouts.py, and ~root-header/~root-mobile
wrapper defcomps to layout.sx. Convert built-in "root" layout to .sx.

Phases 1-3: Convert account (65→19 lines), federation (105→97 lines),
and orders (88→21 lines) to use register_sx_layout with .sx defcomps
that read ctx values as free variables from the evaluation environment.
No more Python building SX strings via SxExpr(await root_header_sx(ctx)).

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

orders/sx/layouts.sx

@@ -0,0 +1,49 @@
+;; Orders layout defcomps — read ctx values from env free variables.
+;; Registered via register_sx_layout("orders", ...) in __init__.py.
+
+;; --- orders layout: root + auth + orders rows ---
+
+(defcomp ~orders-layout-full (&key list-url)
+  (<> (~root-header)
+    (~header-child-sx
+      :inner (<> (~auth-header-row :account-url account-url
+                                    :select-colours select-colours
+                                    :account-nav account-nav)
+               (~orders-header-row :list-url (or list-url "/"))))))
+
+(defcomp ~orders-layout-oob (&key list-url)
+  (<> (~auth-header-row :account-url account-url
+                         :select-colours select-colours
+                         :account-nav account-nav
+                         :oob true)
+    (~oob-header-sx
+      :parent-id "auth-header-child"
+      :row (~orders-header-row :list-url (or list-url "/")))
+    (~root-header :oob true)))
+
+(defcomp ~orders-layout-mobile ()
+  (~root-mobile))
+
+;; --- order-detail layout: root + auth + orders + order rows ---
+
+(defcomp ~order-detail-layout-full (&key list-url detail-url)
+  (<> (~root-header)
+    (~order-detail-header-stack
+      :auth (~auth-header-row :account-url account-url
+                               :select-colours select-colours
+                               :account-nav account-nav)
+      :orders (~orders-header-row :list-url (or list-url "/"))
+      :order (~menu-row-sx :id "order-row" :level 3 :colour "sky"
+               :link-href (or detail-url "/") :link-label "Order"
+               :icon "fa fa-gbp"))))
+
+(defcomp ~order-detail-layout-oob (&key detail-url)
+  (<> (~oob-header-sx
+        :parent-id "orders-header-child"
+        :row (~menu-row-sx :id "order-row" :level 3 :colour "sky"
+               :link-href (or detail-url "/") :link-label "Order"
+               :icon "fa fa-gbp" :oob true))
+    (~root-header :oob true)))
+
+(defcomp ~order-detail-layout-mobile ()
+  (~root-mobile))