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 && (