feat(desktop): show the installed app's own version in the UI

Nothing displayed which desktop build was actually installed — debugging
a stuck update meant inferring the current version backwards from the
update prompt's target version. Added DesktopVersionBadge (next to the
existing server-side VersionBadge) using @tauri-apps/api's getVersion(),
the real running app version baked in from tauri.conf.json. No-ops in a
browser. Exported inTauri() from origin.ts instead of redefining it again.

apps/web/src/lib/origin.ts

@@ -39,7 +39,7 @@ export function wsUrl(path: string): string {
 }
 
 /** True when running inside the Tauri webview (not a normal browser). */
-function inTauri(): boolean {
+export function inTauri(): boolean {
   return typeof window !== "undefined" && "__TAURI_INTERNALS__" in window;
 }