Move SX app CSS and init behavior from Python to init.sx

Styles (indicator, jiggle animation) and nav aria-selected behavior
were inline Python strings in sx/app.py. Now they live in sx/sx/init.sx
as proper SX source — styles via collect! "cssx", nav via dom-listen.

The shell's inline_css is empty; CSSX handles style injection on boot.

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

sx/sx/init.sx

@@ -0,0 +1,24 @@
+;; ---------------------------------------------------------------------------
+;; SX app boot — styles and behaviors injected on page load
+;;
+;; Replaces inline_css and init_sx from Python app config.
+;; Called as a data-init script on every page.
+;; ---------------------------------------------------------------------------
+
+;; Framework styles — request indicators + link jiggle
+(collect! "cssx" ".sx-indicator{display:none}")
+(collect! "cssx" ".sx-request .sx-indicator{display:inline-flex}")
+(collect! "cssx" "@keyframes sxJiggle{0%,100%{transform:translateX(0)}25%{transform:translateX(-.5px)}75%{transform:translateX(.5px)}}")
+(collect! "cssx" "a.sx-request{animation:sxJiggle .3s ease-in-out infinite}")
+
+;; Nav link aria-selected update on client-side routing
+(dom-listen (dom-body) "sx:clientRoute"
+  (fn (e)
+    (let ((p (get (event-detail e) "pathname")))
+      (when p
+        (for-each
+          (fn (a) (dom-set-attr a "aria-selected" "false"))
+          (dom-query-all "nav a[aria-selected]"))
+        (for-each
+          (fn (a) (dom-set-attr a "aria-selected" "true"))
+          (dom-query-all (str "nav a[href=\"" p "\"]")))))))