fix(ui): add independent scroll containers to fix scroll wheel on Linux

Providers page was using DndContext which may interfere with scroll wheel
events on Linux/Ubuntu WebKitGTK. Added independent scroll containers
with `overflow-y-auto` to all main pages, matching the pattern already
used by the MCP panel which works correctly.

Changes:
- App.tsx: Wrap ProviderList in independent scroll container
- SkillsPage: Use consistent h-[calc(100vh-8rem)] layout
- SettingsPage: Add overflow-hidden and overflow-x-hidden for consistency

src/components/settings/SettingsPage.tsx

@@ -151,7 +151,7 @@ export function SettingsPage({
   const isBusy = useMemo(() => isLoading && !settings, [isLoading, settings]);
 
   return (
-    <div className="mx-auto max-w-[56rem] flex flex-col h-[calc(100vh-8rem)] px-6">
+    <div className="mx-auto max-w-[56rem] flex flex-col h-[calc(100vh-8rem)] overflow-hidden px-6">
       {isBusy ? (
         <div className="flex flex-1 items-center justify-center">
           <Loader2 className="h-8 w-8 animate-spin text-muted-foreground" />
@@ -172,7 +172,7 @@ export function SettingsPage({
             <TabsTrigger value="about">{t("common.about")}</TabsTrigger>
           </TabsList>
 
-          <div className="flex-1 overflow-y-auto pr-2">
+          <div className="flex-1 overflow-y-auto overflow-x-hidden pr-2">
             <TabsContent value="general" className="space-y-6 mt-0">
               {settings ? (
                 <>