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/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::*;