Prefix all SX URLs with /sx/ for WhatsApp-safe sharing

All routes moved under /sx/ prefix:
- / redirects to /sx/
- /sx/ serves home page
- /sx/<path:expr> is the catch-all for SX expression URLs
- Bare /(...) and /~... redirect to /sx/(...) and /sx/~...
- All ~600 hrefs, sx-get attrs, defhandler paths, redirect
  targets, and blueprint routes updated across 44 files

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

sx/sx/not-found.sx

@@ -11,8 +11,8 @@
       (p :class "mt-2"
          :style (cssx (:text (colour "stone" 400) (size "sm") (family "mono")))
         path))
-    (a :href "/"
-       :sx-get "/" :sx-target "#main-panel" :sx-select "#main-panel"
+    (a :href "/sx/"
+       :sx-get "/sx/" :sx-target "#main-panel" :sx-select "#main-panel"
        :sx-swap "outerHTML" :sx-push-url "true"
        :class "inline-block mt-6 px-4 py-2 rounded border transition-colors"
        :style (cssx (:text (colour "violet" 700) (size "sm"))