Nav Back: track popstate delta so back-then-forward keeps accurate depth

Codex's remaining caveat: appNavDepth decremented on every popstate, so a
browser Back then Forward undercounted (in-page Back would jump to Highlights
early). Use the navigation's signed delta on popstate (Back -1, Forward +1,
±N for jumps) instead of a flat decrement, so the depth stays accurate through
any back/forward dance. Falls back to -1 if delta is unavailable (safe).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -276,10 +276,11 @@
   // The initial 'enter' is handled by onMount, after its data deps are fetched.
   afterNavigate((nav) => {
     if (nav.type === 'enter') return;
-    // Forward in-app nav deepens the stack; back/forward (popstate) unwinds it.
-    // Clamped at 0, so an out-of-app landing keeps Back app-safe (→ Highlights).
+    // Forward in-app nav deepens the stack; popstate moves by its signed delta
+    // (Back = -1, Forward = +1, ±N for jumps), so a back-then-forward dance keeps
+    // an accurate count. Clamped at 0, so an out-of-app landing stays app-safe.
     if (nav.type === 'goto' || nav.type === 'link') appNavDepth += 1;
-    else if (nav.type === 'popstate') appNavDepth = Math.max(0, appNavDepth - 1);
+    else if (nav.type === 'popstate') appNavDepth = Math.max(0, appNavDepth + (nav.delta ?? -1));
     loadView(selected);
   });