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>

account/sx/layouts.sx

@@ -0,0 +1,27 @@
+;; Account layout defcomps — read ctx values from env free variables.
+;; Registered via register_sx_layout("account", ...) in __init__.py.
+
+;; Full page: root header + auth header row in header-child
+(defcomp ~account-layout-full ()
+  (<> (~root-header)
+    (~header-child-sx
+      :inner (~auth-header-row :account-url account-url
+                                :select-colours select-colours
+                                :account-nav account-nav))))
+
+;; OOB (HTMX): auth row + root header, both with oob=true
+(defcomp ~account-layout-oob ()
+  (<> (~auth-header-row :account-url account-url
+                         :select-colours select-colours
+                         :account-nav account-nav
+                         :oob true)
+    (~root-header :oob true)))
+
+;; Mobile menu: auth section + root nav
+(defcomp ~account-layout-mobile ()
+  (<> (~mobile-menu-section
+        :label "account" :href "/" :level 1 :colour "sky"
+        :items (~auth-nav-items :account-url account-url
+                                 :select-colours select-colours
+                                 :account-nav account-nav))
+    (~root-mobile)))