Phase 7: Replace render_template() with s-expression rendering in all POST/PUT/DELETE routes

Eliminates all render_template() calls from POST/PUT/DELETE handlers across
all 7 services. Moves sexp_components.py into sexp/ packages per service.

- Blog: like toggle, snippets, cache clear, features/sumup/entry panels,
  create/delete market, WYSIWYG editor panel (render_editor_panel)
- Federation: like/unlike/boost/unboost, follow/unfollow, actor card,
  interaction buttons
- Events: ticket widget, checkin, confirm/decline/provisional, tickets
  config, posts CRUD, description edit/save, calendar/slot/ticket_type
  CRUD, payments, buy tickets, day main panel, entry page
- Market: like toggle, cart add response
- Account: newsletter toggle
- Cart: checkout error pages (3 handlers)
- Orders: checkout error page (1 handler)

Remaining render_template() calls are exclusively in GET handlers and
internal services (email templates, fragment endpoints).

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

federation/bp/identity/routes.py

@@ -8,7 +8,7 @@ from __future__ import annotations
 import re
 
 from quart import (
-    Blueprint, request, render_template, redirect, url_for, g, abort,
+    Blueprint, request, redirect, url_for, g, abort,
 )
 
 from shared.services.registry import services
@@ -40,7 +40,7 @@ def register(url_prefix="/identity"):
             return redirect(url_for("activitypub.actor_profile", username=actor.preferred_username))
 
         from shared.sexp.page import get_template_context
-        from sexp_components import render_choose_username_page
+        from sexp.sexp_components import render_choose_username_page
         ctx = await get_template_context()
         ctx["actor"] = actor
         return await render_choose_username_page(ctx)
@@ -71,11 +71,11 @@ def register(url_prefix="/identity"):
             error = "This username is already taken."
 
         if error:
-            return await render_template(
-                "federation/choose_username.html",
-                error=error,
-                username=username,
-            ), 400
+            from shared.sexp.page import get_template_context
+            from sexp.sexp_components import render_choose_username_page
+            ctx = await get_template_context(error=error, username=username)
+            ctx["actor"] = None
+            return await render_choose_username_page(ctx), 400
 
         # Create ActorProfile with RSA keys
         display_name = g.user.name or username