Move SX construction from Python to .sx defcomps (phases 0-4)

Eliminate Python s-expression string building across account, orders,
federation, and cart services. Visual rendering logic now lives entirely
in .sx defcomp components; Python files contain only data serialization,
header/layout wiring, and thin wrappers that call defcomps.

Phase 0: Shared DRY extraction — auth/orders header defcomps, format-decimal/
pluralize/escape/route-prefix primitives.
Phase 1: Account — dashboard, newsletters, login/device/check-email content.
Phase 2: Orders — order list, detail, filter, checkout return assembled defcomps.
Phase 3: Federation — social nav, post cards, timeline, search, actors,
notifications, compose, profile assembled defcomps.
Phase 4: Cart — overview, page cart items/calendar/tickets/summary, admin,
payments assembled defcomps; orders rendering reuses Phase 2 shared defcomps.

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

shared/sx/templates/auth.sx

@@ -1,5 +1,44 @@
-;; Shared auth components — login flow, check email
-;; Used by account and federation services.
+;; Shared auth components — login flow, check email, header rows
+;; Used by account, orders, cart, and federation services.
+
+;; ---------------------------------------------------------------------------
+;; Auth / orders header rows — DRY extraction from per-service Python
+;; ---------------------------------------------------------------------------
+
+;; Auth section nav items (newsletters link + account_nav slot)
+(defcomp ~auth-nav-items (&key account-url select-colours account-nav)
+  (<>
+    (~nav-link :href (str (or account-url "") "/newsletters/")
+               :label "newsletters"
+               :select-colours (or select-colours ""))
+    (when account-nav account-nav)))
+
+;; Auth header row — wraps ~menu-row-sx for account section
+(defcomp ~auth-header-row (&key account-url select-colours account-nav oob)
+  (~menu-row-sx :id "auth-row" :level 1 :colour "sky"
+    :link-href (str (or account-url "") "/")
+    :link-label "account" :icon "fa-solid fa-user"
+    :nav (~auth-nav-items :account-url account-url
+                          :select-colours select-colours
+                          :account-nav account-nav)
+    :child-id "auth-header-child" :oob oob))
+
+;; Auth header row without nav (for cart service)
+(defcomp ~auth-header-row-simple (&key account-url oob)
+  (~menu-row-sx :id "auth-row" :level 1 :colour "sky"
+    :link-href (str (or account-url "") "/")
+    :link-label "account" :icon "fa-solid fa-user"
+    :child-id "auth-header-child" :oob oob))
+
+;; Orders header row
+(defcomp ~orders-header-row (&key list-url)
+  (~menu-row-sx :id "orders-row" :level 2 :colour "sky"
+    :link-href list-url :link-label "Orders" :icon "fa fa-gbp"
+    :child-id "orders-header-child"))
+
+;; ---------------------------------------------------------------------------
+;; Auth forms — login flow, check email
+;; ---------------------------------------------------------------------------
 
 (defcomp ~auth-error-banner (&key error)
   (when error