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/locales/en.ts

@@ -540,6 +540,8 @@ export const enTranslation = {
     discordRichPresenceDesc: 'Show the currently playing track on your Discord profile. Requires Discord to be running.',
     useCustomTitlebar: 'Custom title bar',
     useCustomTitlebarDesc: 'Replace the system title bar with a built-in one that matches the app theme. Disable to use the native GNOME/GTK title bar.',
+    linuxWebkitSmoothScroll: 'Smooth wheel (Linux)',
+    linuxWebkitSmoothScrollDesc: 'On: inertial scroll. Off: line-by-line, GTK-style.',
     discordAppleCovers: 'Fetch covers from Apple Music for Discord',
     discordAppleCoversDesc: 'Sends the artist and album name to Apple\'s search API to find cover art for your Discord profile. Disabled by default for privacy.',
     discordOptions: 'Advanced Discord options',