Windows port: cfg-gate iced window application_id (Linux-only field)

iced's `window::settings::PlatformSpecific::application_id` only exists on
Linux (X11/Wayland use it to match the .desktop launcher icon); on Windows
the struct exposes a different field set, so the unconditional assignment
failed to compile for `*-pc-windows-*`. This was the first real Windows
compile blocker surfaced now that the port actually cross-compiles.

Move the field behind a `platform_specific_settings()` helper gated on
`target_os = "linux"`, with a defaults-only variant elsewhere. Linux build
unchanged (verified `cargo check`); the windows-gnu target now builds a
runnable .exe (verified launching under Wine: GUI renders, iroh network
stack + ring identity init, config/identity land in %APPDATA%).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -553,11 +553,9 @@ pub fn run_gui() -> iced::Result {
             // the icon from the .desktop file matched by app_id instead).
             icon: window_icon(),
             // app_id must match the .desktop basename so Wayland compositors
-            // (e.g. KWin) attach our launcher icon to the window.
-            platform_specific: iced::window::settings::PlatformSpecific {
-                application_id: "peerspeak".to_string(),
-                ..Default::default()
-            },
+            // (e.g. KWin) attach our launcher icon to the window. The field is
+            // Linux-only in iced (X11/Wayland); see platform_specific_settings().
+            platform_specific: platform_specific_settings(),
             // We save the final size ourselves on CloseRequested, then exit.
             exit_on_close_request: false,
             ..Default::default()
@@ -565,6 +563,22 @@ pub fn run_gui() -> iced::Result {
         .run()
 }
 
+/// Window `PlatformSpecific` settings. `application_id` (used by X11/Wayland to
+/// match our `.desktop` launcher icon) only exists in iced on Linux, so it is
+/// set there and left at defaults on Windows.
+#[cfg(target_os = "linux")]
+fn platform_specific_settings() -> iced::window::settings::PlatformSpecific {
+    iced::window::settings::PlatformSpecific {
+        application_id: "peerspeak".to_string(),
+        ..Default::default()
+    }
+}
+
+#[cfg(not(target_os = "linux"))]
+fn platform_specific_settings() -> iced::window::settings::PlatformSpecific {
+    iced::window::settings::PlatformSpecific::default()
+}
+
 /// Build the window icon from an embedded 128×128 straight-RGBA blob rendered
 /// from `assets/icons/peerspeak.svg`. Using `from_rgba` (always available) keeps
 /// us off iced's heavy `image` feature — the blob is raw pixels, no decoder.