build(windows): embed the Goblin icon into goblin.exe via winresource

The .msi shortcuts already carry the icon, but the bare goblin.exe had none, so
Explorer/taskbar showed the generic exe icon. build.rs now embeds
wix/Product.ico (the yellow Goblin icon) as the exe's application icon resource.

Gated to Windows hosts: winresource is a `cfg(windows)` build-dependency and the
embed fn is `#[cfg(windows)]` (with a no-op stub otherwise), so Linux/macOS/
Android builds don't compile or run it — other releases are untouched. The embed
is best-effort (warns, never fails the build) in case rc.exe is unavailable.

Cargo.toml

@@ -171,6 +171,12 @@ eframe = { version = "0.33.2", default-features = false, features = ["glow", "an
 [build-dependencies]
 built = "0.8.0"
 
+# Windows hosts only: embed the Goblin icon (wix/Product.ico) into goblin.exe via
+# build.rs. Not compiled on Linux/macOS/Android hosts, so other builds are
+# unaffected.
+[target.'cfg(windows)'.build-dependencies]
+winresource = "0.1"
+
 [dev-dependencies]
 nostr-sdk = { version = "0.44", features = ["nip06", "nip44", "nip49", "nip59", "nip98"] }
 tokio = { version = "1.49.0", features = ["full"] }