Enable cross-subdomain htmx and purify layout to sexp

- Disable htmx selfRequestsOnly, add CORS headers for *.rose-ash.com
- Remove same-origin guards from ~menu-row and ~nav-link htmx attrs
- Convert ~app-layout from string-concatenated HTML to pure sexp tree
- Extract ~app-head component, replace ~app-shell with inline structure
- Convert hamburger SVG from Python HTML constant to ~hamburger sexp component
- Fix cross-domain fragment URLs (events_url, market_url)
- Fix starts-with? primitive to handle nil values
- Fix duplicate admin menu rows on OOB swaps
- Add calendar admin nav links (slots, description)
- Convert slots page from Jinja to sexp rendering
- Disable page caching in development mode
- Backfill migration to clean orphaned container_relations

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

shared/sexp/primitives.py

@@ -255,7 +255,9 @@ def prim_join(sep: str, coll: list) -> str:
     return sep.join(str(x) for x in coll)
 
 @register_primitive("starts-with?")
-def prim_starts_with(s: str, prefix: str) -> bool:
+def prim_starts_with(s, prefix: str) -> bool:
+    if not isinstance(s, str):
+        return False
     return s.startswith(prefix)
 
 @register_primitive("ends-with?")