feat(linux): add WebKitGTK smooth wheel scroll toggle in settings

Persist preference in auth store, sync from App on Linux, and expose a
Tauri command using webkit2gtk to toggle enable-smooth-scrolling at
runtime. Default on to match upstream; users may disable for discrete
GTK-style line steps.

Apply a one-time rehydrate migration so smooth scrolling stays on after
updates even if an older build persisted the wrong default.

src-tauri/Cargo.toml

@@ -54,6 +54,8 @@ libc = "0.2"
 
 [target.'cfg(target_os = "linux")'.dependencies]
 zbus = { version = "5.9", default-features = false, features = ["blocking-api"] }
+# Match wry/tauri’s WebKitGTK stack — used only to turn off kinetic wheel scrolling.
+webkit2gtk = { version = "2.0", default-features = false, features = ["v2_40"] }
 
 [target.'cfg(windows)'.dependencies]
 windows = { version = "0.58", features = [