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/app.py

@@ -119,15 +119,23 @@ def create_app() -> "Quart":
 
     @app.before_request
     async def sx_url_redirect():
-        """Redirect old-style paths to SX expression URLs (301)."""
+        """Redirect old-style paths and bare root to /sx/ URLs (301)."""
         from quart import request, redirect as q_redirect
         path = request.path
+        # Root → /sx/
+        if path == "/":
+            return q_redirect("/sx/", 301)
         # Skip non-page paths
-        if path.startswith(("/static/", "/internal/", "/auth/", "/sx/")):
+        if path.startswith(("/static/", "/internal/", "/auth/")):
             return None
-        # Skip SX expression URLs (already in new format)
+        # Skip SX expression URLs (already in new format under /sx/)
+        if path.startswith("/sx/"):
+            return None
+        # Redirect bare /(...) to /sx/(...)
         if path.startswith("/(") or path.startswith("/~"):
-            return None
+            qs = request.query_string.decode()
+            target = f"/sx{path}" + ("?" + qs if qs else "")
+            return q_redirect(target, 301)
         new_url = redirect_old_url(path)
         if new_url:
             qs = request.query_string.decode()
@@ -140,20 +148,26 @@ def create_app() -> "Quart":
         from quart import request, redirect
         path = request.path
         # Skip SX expression URLs — they don't use trailing slashes
-        if "(" in path or path.startswith("/~"):
+        if "(" in path or "/~" in path:
             return None
         if (path != "/"
+                and path != "/sx/"
                 and not path.endswith("/")
                 and request.method == "GET"
-                and not path.startswith(("/static/", "/internal/", "/auth/"))
+                and not path.startswith(("/static/", "/internal/", "/auth/", "/sx/"))
                 and "." not in path.rsplit("/", 1)[-1]):
             qs = request.query_string.decode()
             target = path + "/" + ("?" + qs if qs else "")
             return redirect(target, 301)
 
-    @app.get("/<path:expr>")
+    @app.get("/sx/")
+    async def sx_home():
+        """SX docs home page."""
+        return await eval_sx_url("/")
+
+    @app.get("/sx/<path:expr>")
     async def sx_eval_route(expr):
-        """Catch-all: evaluate SX expression URLs."""
+        """Catch-all: evaluate SX expression URLs under /sx/ prefix."""
         result = await eval_sx_url(f"/{expr}")
         if result is None:
             from quart import abort