fix: align title bar headers across panels, fix macOS Dock icon size

Title bar alignment (AppShell.tsx, page.tsx, DocPreview.tsx):
- Reduce AppShell draggable spacer from h-11 to h-5 to match side panels' mt-5
- Chat title bar uses normal flow h-12 instead of absolute positioning hack
- DocPreview header changed from h-10 to h-12 mt-5
- All three columns now align at the same vertical position

macOS Dock icon (build/icon.png, build/icon.icns):
- Add 100px transparent padding on all sides (content 824x824 in 1024x1024 canvas)
- Follows Apple Human Interface Guidelines for macOS app icons
- Regenerated icon.icns with all required sizes
- Original icon backed up as icon-original.png

Remove start execution button (ChatView.tsx):
- Remove showStartExecution logic and handleStartExecution callback
- Mode switching is now handled automatically by SDK

src/app/chat/[id]/page.tsx

@@ -166,7 +166,7 @@ export default function ChatSessionPage({ params }: ChatSessionPageProps) {
       {/* Chat title bar */}
       {sessionTitle && (
         <div
-          className="flex items-center justify-center px-4 pb-2 gap-1"
+          className="flex h-12 shrink-0 items-center justify-center px-4 gap-1"
           style={{ WebkitAppRegion: 'drag' } as React.CSSProperties}
         >
           {projectName && (

src/components/chat/ChatView.tsx

@@ -504,17 +504,6 @@ export function ChatView({ sessionId, initialMessages = [], initialHasMore = fal
     }
   }, [sessionId, sendMessage]);
 
-  // Determine if "Start Execution" button should show
-  const showStartExecution = mode === 'plan' && !isStreaming && messages.length > 0 && messages[messages.length - 1]?.role === 'assistant';
-
-  const handleStartExecution = useCallback(() => {
-    handleModeChange('code');
-    // Small delay to let mode change propagate before sending
-    setTimeout(() => {
-      sendMessage('请按照上面的计划开始执行。');
-    }, 100);
-  }, [handleModeChange, sendMessage]);
-
   return (
     <div className="flex h-full min-h-0 flex-col">
       <MessageList
@@ -533,17 +522,6 @@ export function ChatView({ sessionId, initialMessages = [], initialHasMore = fal
         loadingMore={loadingMore}
         onLoadMore={loadEarlierMessages}
       />
-      {showStartExecution && (
-        <div className="flex justify-center py-3 px-4">
-          <button
-            onClick={handleStartExecution}
-            className="inline-flex items-center gap-2 rounded-lg bg-primary px-4 py-2 text-sm font-medium text-primary-foreground shadow-sm transition-colors hover:bg-primary/90"
-          >
-            <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round"><polygon points="5 3 19 12 5 21 5 3"/></svg>
-            开始执行
-          </button>
-        </div>
-      )}
       <MessageInput
         onSend={sendMessage}
         onCommand={handleCommand}

src/components/layout/AppShell.tsx

@@ -258,9 +258,9 @@ export function AppShell({ children }: { children: React.ReactNode }) {
               <ResizeHandle side="left" onResize={handleChatListResize} onResizeEnd={handleChatListResizeEnd} />
             )}
             <div className="flex min-w-0 flex-1 flex-col overflow-hidden">
-              {/* Electron draggable title bar region */}
+              {/* Electron draggable title bar region — matches side panels' mt-5 */}
               <div
-                className="h-11 w-full shrink-0"
+                className="h-5 w-full shrink-0"
                 style={{ WebkitAppRegion: 'drag' } as React.CSSProperties}
               />
               <main className="relative flex-1 overflow-hidden">{children}</main>

src/components/layout/DocPreview.tsx

@@ -120,7 +120,7 @@ export function DocPreview({
       style={{ width }}
     >
       {/* Header */}
-      <div className="flex h-10 shrink-0 items-center gap-2 px-3">
+      <div className="flex h-12 mt-5 shrink-0 items-center gap-2 px-3">
         <div className="min-w-0 flex-1">
           <p className="truncate text-sm font-medium">{fileName}</p>
         </div>