Server cleanup: extract app-specific config into SX

Phase 1 Step 2 of architecture roadmap. The OCaml HTTP server is now
generic — all sx_docs-specific values (layout components, path prefix,
title, warmup paths, handler prefixes, CSS/JS, client libs) move into
sx/sx/app-config.sx as a __app-config dict. Server reads config at
startup with hardcoded defaults as fallback, so it works with no config,
partial config, or full config.

Removed: 9 demo data stubs, stepper cookie cache logic, page-functions.sx
directory heuristic. Added: 29-test server config test suite covering
standard, custom, no-config, and minimal-config scenarios.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/request-handler.sx

@@ -2,14 +2,18 @@
   sx-url-to-expr
   (fn
     (path)
-    (cond
-      (or (= path "/") (= path "/sx/") (= path "/sx"))
-      "home"
-      (starts-with? path "/sx/")
-      (join " " (split (slice path 4 (len path)) "."))
-      (starts-with? path "/")
-      (join " " (split (slice path 1 (len path)) "."))
-      :else path)))
+    (let
+      ((prefix (cek-try (fn () (or (dict-get __app-config :path-prefix) "/sx/")) (fn (e) "/sx/")))
+        (prefix-len (len prefix))
+        (prefix-bare (slice prefix 0 (- prefix-len 1))))
+      (cond
+        (or (= path "/") (= path prefix) (= path prefix-bare))
+        "home"
+        (starts-with? path prefix)
+        (join " " (split (slice path prefix-len (len path)) "."))
+        (starts-with? path "/")
+        (join " " (split (slice path 1 (len path)) "."))
+        :else path))))
 
 (define
   sx-auto-quote
@@ -52,5 +56,13 @@
         (nil? page-ast)
         nil
         (let
-          ((nav-path (if (and (>= (len path) 4) (= (slice path 0 4) "/sx/")) path (str "/sx" path))))
+          ((prefix (cek-try (fn () (or (dict-get __app-config :path-prefix) "/sx/")) (fn (e) "/sx/")))
+            (prefix-len (len prefix))
+            (nav-path
+              (if
+                (and
+                  (>= (len path) prefix-len)
+                  (= (slice path 0 prefix-len) prefix))
+                path
+                (str (slice prefix 0 (- prefix-len 1)) path))))
           {:page-ast page-ast :nav-path nav-path :is-ajax is-ajax})))))