Send all responses as sexp wire format with client-side rendering

- Server sends sexp source text, client (sexp.js) renders everything
- SexpExpr marker class for nested sexp composition in serialize()
- sexp_page() HTML shell with data-mount="body" for full page loads
- sexp_response() returns text/sexp for OOB/partial responses
- ~app-body layout component replaces ~app-layout (no raw!)
- ~rich-text is the only component using raw! (for CMS HTML content)
- Fragment endpoints return text/sexp, auto-wrapped in SexpExpr
- All _*_html() helpers converted to _*_sexp() returning sexp source
- Head auto-hoist: sexp.js moves meta/title/link/script[ld+json]
  from rendered body to document.head automatically
- Unknown components render warning box instead of crashing page
- Component kwargs preserve AST for lazy rendering (fixes <> in kwargs)
- Fix unterminated paren in events/sexp/tickets.sexpr

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

shared/sexp/jinja_bridge.py

@@ -191,7 +191,7 @@ def _get_request_context():
     try:
         from quart import g, request
         user = getattr(g, "user", None)
-        is_htmx = bool(request.headers.get("HX-Request"))
+        is_htmx = bool(request.headers.get("SX-Request") or request.headers.get("HX-Request"))
         return RequestContext(user=user, is_htmx=is_htmx)
     except Exception:
         return RequestContext()