fix: add scrollbars to provider dialogs and simplify theme toggle

## Changes

### Add scrollbars to provider dialogs
- **AddProviderDialog.tsx**: Add max-h-[90vh], flex flex-col layout, and scrollable content wrapper
- **EditProviderDialog.tsx**: Add max-h-[90vh], flex flex-col layout, and scrollable content wrapper
- Both dialogs now follow the same scrolling pattern as other dialogs in the app
- Wrap ProviderForm in `<div className="flex-1 overflow-y-auto -mx-6 px-6">` for proper scrolling

### Simplify theme toggle
- **mode-toggle.tsx**: Change from dropdown menu to direct toggle button
- Remove DropdownMenu and related imports
- Click now directly toggles between light and dark mode
- Simpler UX: one click to switch themes instead of opening a menu
- Remove "system" theme option from quick toggle (still available in settings if needed)

## Benefits
- **Consistent scrolling**: All dialogs now have proper scroll behavior when content exceeds viewport height
- **Better UX**: Theme toggle is faster and more intuitive with direct click
- **Code simplification**: Removed unnecessary dropdown menu complexity from theme toggle

All TypeScript type checks and Prettier formatting checks pass.

src/components/providers/AddProviderDialog.tsx

@@ -56,7 +56,7 @@ export function AddProviderDialog({
 
   return (
     <Dialog open={open} onOpenChange={onOpenChange}>
-      <DialogContent className="max-w-2xl">
+      <DialogContent className="max-w-2xl max-h-[90vh] flex flex-col">
         <DialogHeader>
           <DialogTitle>{submitLabel}</DialogTitle>
           <DialogDescription>
@@ -66,12 +66,14 @@ export function AddProviderDialog({
           </DialogDescription>
         </DialogHeader>
 
-        <ProviderForm
-          appType={appType}
-          submitLabel={t("common.add", { defaultValue: "添加" })}
-          onSubmit={handleSubmit}
-          onCancel={() => onOpenChange(false)}
-        />
+        <div className="flex-1 overflow-y-auto -mx-6 px-6">
+          <ProviderForm
+            appType={appType}
+            submitLabel={t("common.add", { defaultValue: "添加" })}
+            onSubmit={handleSubmit}
+            onCancel={() => onOpenChange(false)}
+          />
+        </div>
       </DialogContent>
     </Dialog>
   );