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/package.json

@@ -21,6 +21,7 @@
     "vue-router": "^4.4.3"
   },
   "devDependencies": {
+    "@types/node": "^22.7.4",
     "typescript": "^5.6.2"
   }
 }