fix: warn before discarding unsaved changes in waveconfig view (#3099)

Fixes #2890

## Changes

Add unsaved changes confirmation when navigating away in the Wave Config
view. Two navigation paths lose edits without warning:

1. **File sidebar selection**: `handleFileSelect` calls
`model.loadFile(file)` directly, which resets `hasEditedAtom` and
overwrites `fileContentAtom` — discarding edits silently.

2. **JSON→Visual tab switching**: `setActiveTab()` is called directly.
The visual component renders from saved config (not the editor buffer),
so edits appear lost.

### Fix

- Added `confirmDiscardChanges()` to `WaveConfigViewModel` — checks
`hasEditedAtom`, shows `window.confirm()` if there are unsaved changes.
- Guarded `handleFileSelect` with the confirmation check (skips if
already on the selected file).
- Guarded JSON→Visual tab switch with the confirmation check.

Only prompts when `hasEditedAtom` is true. Visual→JSON direction is safe
(editor loads from `fileContentAtom`). `window.confirm()` is synchronous
and works well in Electron.

## Test Plan

- Edit JSON, switch file in sidebar → confirmation dialog appears.
Cancel stays, confirm switches.
- Edit JSON, click Visual tab → confirmation dialog appears.
- Save changes first, then switch → no dialog.
- Switch without editing → no dialog.
- `npx tsc --noEmit` passes.

---------

Signed-off-by: majiayu000 <1835304752@qq.com>
Co-authored-by: sawka <mike@commandline.dev>

Author: majiayu000

frontend/app/view/waveconfig/waveconfig-model.ts

@@ -263,6 +263,21 @@ export class WaveConfigViewModel implements ViewModel {
         return globalStore.get(this.hasEditedAtom);
     }
 
+    confirmDiscardChanges(): boolean {
+        if (!this.hasChanges()) {
+            return true;
+        }
+        return window.confirm("You have unsaved changes. Discard and continue?");
+    }
+
+    discardChanges() {
+        const originalContent = globalStore.get(this.originalContentAtom);
+        globalStore.set(this.fileContentAtom, originalContent);
+        globalStore.set(this.hasEditedAtom, false);
+        globalStore.set(this.validationErrorAtom, null);
+        globalStore.set(this.errorMessageAtom, null);
+    }
+
     markAsEdited() {
         globalStore.set(this.hasEditedAtom, true);
     }

frontend/app/view/waveconfig/waveconfig.tsx

@@ -26,6 +26,8 @@ const ConfigSidebar = memo(({ model }: ConfigSidebarProps) => {
     const configErrorFiles = useAtomValue(model.configErrorFilesAtom);
 
     const handleFileSelect = (file: ConfigFile) => {
+        if (selectedFile?.path === file.path) return;
+        if (!model.confirmDiscardChanges()) return;
         model.loadFile(file);
         setIsMenuOpen(false);
     };
@@ -231,7 +233,11 @@ const WaveConfigView = memo(({ blockId, model }: ViewComponentProps<WaveConfigVi
                             {selectedFile.visualComponent && selectedFile.hasJsonView && (
                                 <div className="flex gap-0 border-b border-border">
                                     <button
-                                        onClick={() => setActiveTab("visual")}
+                                        onClick={() => {
+                                            if (!model.confirmDiscardChanges()) return;
+                                            model.discardChanges();
+                                            setActiveTab("visual");
+                                        }}
                                         className={cn(
                                             "px-4 pt-1 pb-1.5 cursor-pointer transition-colors text-secondary",
                                             activeTab === "visual"
@@ -241,6 +247,7 @@ const WaveConfigView = memo(({ blockId, model }: ViewComponentProps<WaveConfigVi
                                     >
                                         Visual
                                     </button>
+                                    {/* No guard needed: visual tab saves changes immediately via RPC */}
                                     <button
                                         onClick={() => setActiveTab("json")}
                                         className={cn(