feat(tauri-app): expand tray menu + open_external_url + Profile/Search items

Tray menu now has:
  Show maarcadetweet
  Home
  Compose
  Profile
  Search
  ----
  Quit

The Home/Profile/Search items emit 'app://navigate' events
which the frontend's listenTrayEvents translates to view
switches. The compose and show events continue to be
separate event types ('app://compose', 'app://show').

open_external_url Tauri command takes a URL, validates it's
http(s), and uses tauri-plugin-shell to open it in the user's
default browser. The frontend's openExternalUrl falls back
to window.open in the browser preview (no Tauri runtime).

svelte-check error fix: tauriCall<T>(cmd, fallback, args?)
had the second call argument as 'undefined' instead of 'null'
on the call site for session.load(). The TypeScript compiler
correctly noted that the fallback type 'T' (here Session |
null) couldn't be undefined. Replaced with 'null' and
dropped the trailing null args argument (it's optional).

crates/tauri-app/src/lib/api/client.ts

@@ -51,7 +51,7 @@ function createSessionStore() {
   return {
     subscribe,
     async load() {
-      const s = await tauriCall<Session | null>("current_session", undefined, null);
+      const s = await tauriCall<Session | null>("current_session", null);
       set(s);
     },
     async login(handle: string, password: string) {
@@ -355,18 +355,43 @@ export async function showNotification(
 /// The handler receives the event payload (empty object for these
 /// cases). Returns an unsubscribe function.
 export async function listenTrayEvents(
-  handler: (event: "show" | "compose") => void,
+  handler: (event: "show" | "home" | "compose" | "profile" | "search") => void,
 ): Promise<() => void> {
   const { listen } = await import("@tauri-apps/api/event");
   const unlisteners: Array<() => void> = [];
   const u1 = await listen("app://show", () => handler("show"));
-  const u2 = await listen("app://compose", () => handler("compose"));
-  unlisteners.push(u1, u2);
+  const u2 = await listen("app://navigate", (e) => {
+    handler(e.payload as "home" | "profile" | "search");
+  });
+  const u3 = await listen("app://compose", () => handler("compose"));
+  unlisteners.push(u1, u2, u3);
   return () => {
     for (const u of unlisteners) u();
   };
 }
 
+/// Open an external URL in the user's default browser. The Rust
+/// `open_external_url` command enforces http(s) only; in the
+/// browser preview where no Tauri runtime is present, fall back
+/// to `window.open` and treat a popup-blocker denial as
+/// "fine, user can copy the URL themselves".
+export async function openExternalUrl(url: string): Promise<void> {
+  try {
+    if (isTauri()) {
+      await safeInvoke("open_external_url", { url });
+      return;
+    }
+  } catch (e) {
+    console.warn("open_external_url failed", e);
+  }
+  // Browser fallback (vite dev preview).
+  try {
+    window.open(url, "_blank", "noopener,noreferrer");
+  } catch (e) {
+    console.warn("window.open failed (popup blocker?)", e);
+  }
+}
+
 /// Subscribe to `app://notification` events emitted by the Rust
 /// `show_notification` command. Used to focus the window and
 /// navigate when the user clicks the notification.