refactor(app_api): extract window + WebKitGTK platform tweaks

Lift set_window_decorations + linux_webkit_apply_smooth_scrolling +
set_linux_webkit_smooth_scrolling out of app_api/core.rs into
app_api/platform.rs (~49 LOC). These are platform-tweak Tauri
commands (Linux WebKitGTK settings + Linux native-decoration toggle)
— logically separate from runtime control, telemetry, cli bridge,
or wire-UA setup.

This is the "platform" slice from cucadmuh's plan for splitting
core.rs's mixed concerns. Together with perf and cli_bridge, core.rs
is now down to runtime control (greet, exit_app), logging (3 cmds),
and UA setup — 56 LOC, focused.

src-tauri/src/lib_commands/app_api/core.rs

@@ -11,51 +11,6 @@ pub(crate) fn exit_app(app_handle: tauri::AppHandle) {
     app_handle.exit(0);
 }
 
-/// Toggle native window decorations at runtime (Linux custom title bar opt-out).
-#[tauri::command]
-pub(crate) fn set_window_decorations(enabled: bool, app_handle: tauri::AppHandle) {
-    if let Some(win) = app_handle.get_webview_window("main") {
-        let _ = win.set_decorations(enabled);
-        // Re-enabling native decorations on GTK causes the window manager to
-        // re-stack the window, which drops focus. Bring it back immediately.
-        if enabled {
-            let _ = win.set_focus();
-        }
-    }
-}
-
-/// WebKitGTK: `enable-smooth-scrolling` also drives deferred / kinetic wheel scrolling.
-#[cfg(target_os = "linux")]
-pub(crate) fn linux_webkit_apply_smooth_scrolling(win: &tauri::WebviewWindow, enabled: bool) -> Result<(), String> {
-    win.with_webview(move |platform| {
-        use webkit2gtk::{SettingsExt, WebViewExt};
-        if let Some(settings) = platform.inner().settings() {
-            settings.set_enable_smooth_scrolling(enabled);
-        }
-    })
-    .map_err(|e| e.to_string())
-}
-
-/// Called from the frontend settings toggle (Linux); no-op on other platforms.
-#[tauri::command]
-pub(crate) fn set_linux_webkit_smooth_scrolling(enabled: bool, app_handle: tauri::AppHandle) -> Result<(), String> {
-    #[cfg(target_os = "linux")]
-    {
-        use tauri::Manager;
-        // Each WebviewWindow has its own WebKitGTK Settings — main-only left the
-        // mini player on the default (inertial) wheel until the user toggled again.
-        for label in ["main", "mini"] {
-            if let Some(win) = app_handle.get_webview_window(label) {
-                linux_webkit_apply_smooth_scrolling(&win, enabled)?;
-            }
-        }
-    }
-    #[cfg(not(target_os = "linux"))]
-    {
-        let _ = (enabled, app_handle);
-    }
-    Ok(())
-}
 
 #[tauri::command]
 pub(crate) fn set_logging_mode(mode: String) -> Result<(), String> {

src-tauri/src/lib_commands/app_api/mod.rs

@@ -4,6 +4,7 @@ mod cli_bridge;
 mod core;
 mod navidrome;
 mod perf;
+mod platform;
 mod remote;
 mod integration;
 mod analysis;
@@ -12,6 +13,7 @@ pub(crate) use cli_bridge::*;
 pub(crate) use core::*;
 pub(crate) use navidrome::*;
 pub(crate) use perf::*;
+pub(crate) use platform::*;
 pub(crate) use remote::*;
 pub(crate) use integration::*;
 pub(crate) use analysis::*;

src-tauri/src/lib_commands/app_api/platform.rs

@@ -0,0 +1,49 @@
+//! Native-window + WebKitGTK platform tweaks exposed as Tauri commands.
+
+use super::*;
+
+/// Toggle native window decorations at runtime (Linux custom title bar opt-out).
+#[tauri::command]
+pub(crate) fn set_window_decorations(enabled: bool, app_handle: tauri::AppHandle) {
+    if let Some(win) = app_handle.get_webview_window("main") {
+        let _ = win.set_decorations(enabled);
+        // Re-enabling native decorations on GTK causes the window manager to
+        // re-stack the window, which drops focus. Bring it back immediately.
+        if enabled {
+            let _ = win.set_focus();
+        }
+    }
+}
+
+/// WebKitGTK: `enable-smooth-scrolling` also drives deferred / kinetic wheel scrolling.
+#[cfg(target_os = "linux")]
+pub(crate) fn linux_webkit_apply_smooth_scrolling(win: &tauri::WebviewWindow, enabled: bool) -> Result<(), String> {
+    win.with_webview(move |platform| {
+        use webkit2gtk::{SettingsExt, WebViewExt};
+        if let Some(settings) = platform.inner().settings() {
+            settings.set_enable_smooth_scrolling(enabled);
+        }
+    })
+    .map_err(|e| e.to_string())
+}
+
+/// Called from the frontend settings toggle (Linux); no-op on other platforms.
+#[tauri::command]
+pub(crate) fn set_linux_webkit_smooth_scrolling(enabled: bool, app_handle: tauri::AppHandle) -> Result<(), String> {
+    #[cfg(target_os = "linux")]
+    {
+        use tauri::Manager;
+        // Each WebviewWindow has its own WebKitGTK Settings — main-only left the
+        // mini player on the default (inertial) wheel until the user toggled again.
+        for label in ["main", "mini"] {
+            if let Some(win) = app_handle.get_webview_window(label) {
+                linux_webkit_apply_smooth_scrolling(&win, enabled)?;
+            }
+        }
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        let _ = (enabled, app_handle);
+    }
+    Ok(())
+}