Fix desktop crate build; simplify tray/menu and telemetry reconnect

Real bugs found by actually compiling the Tauri crate for the first time:
- Cargo.toml declared a [lib] with no src/lib.rs (cargo refused to parse
  the manifest) — added the standard Tauri 2 main.rs/lib.rs split.
- generate_context!() needs real icon files; icons/ only had a .gitkeep.
  Added placeholder PNG/ICO/ICNS icons.

Also simplified: dropped the tray icon + menu (one more moving part not
needed yet), and the telemetry store's exponential backoff is now a fixed
3s retry, matching the same simplification made to continuum-proxy's
uplink client.

Fixed two real TS bugs while typechecking apps/web for the first time
(missing @types/node for nuxt.config.ts's process.env access, untyped
useFetch call inferring 'never'). Verified: cargo check (desktop),
vue-tsc --noEmit (web + ui packages) all clean.

apps/web/pages/index.vue

@@ -2,10 +2,17 @@
 import { PrinterCard } from "@continuum/ui";
 import { useTelemetryStore } from "~/stores/telemetry";
 
+interface Printer {
+  id: string;
+  name: string;
+  model: string;
+  farmId: string;
+}
+
 const telemetry = useTelemetryStore();
 const config = useRuntimeConfig();
 
-const { data: printers } = await useFetch(`${config.public.apiBase}/printers`, {
+const { data: printers } = await useFetch<Printer[]>(`${config.public.apiBase}/printers`, {
   headers: useRequestHeaders(["cookie"]),
 });
 </script>