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-tauri/Cargo.toml

@@ -22,6 +22,7 @@ tauri-plugin-notification = "2"
 tauri-plugin-dialog = "2"
 tauri-plugin-updater = "2"
 tauri-plugin-window-state = "2"
+tauri-plugin-shell = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tokio = { version = "1", features = ["full"] }