tauri-app: settings view + profile polish

- App.svelte: new 'settings' view with account/actions/about sections,
  sign-out button, pdsBase()/appviewBase() helpers, showError wired
  into logout, profile gains open-in-browser + sign-out + recent-posts
  heading
- NavRail: add settings item + gear icon, View union extended to 5
- tests: NavRail.test.ts now expects 5 buttons + covers settings click,
  NavRailHarness View type extended

crates/tauri-app/src/lib/components/NavRailHarness.svelte

@@ -5,7 +5,7 @@
 
   import NavRail from "./NavRail.svelte";
 
-  type View = "home" | "compose" | "profile" | "search";
+  type View = "home" | "compose" | "profile" | "search" | "settings";
   let view: View = $state("home");
 </script>