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/desktop/src-tauri/Cargo.toml

@@ -9,7 +9,7 @@ rust-version = "1.77"
 tauri-build = { version = "2.0", features = [] }
 
 [dependencies]
-tauri = { version = "2.1", features = ["tray-icon"] }
+tauri = "2.1"
 tauri-plugin-fs = "2.0"
 tauri-plugin-shell = "2.0"
 serde = { version = "1", features = ["derive"] }