Increase max height of chat box (#1957)

I've successfully implemented the change to allow the `ChatBoxBase.tsx` to expand vertically up to 50% of the window height.

**Change made in** `vibe-kanban/frontend/src/components/ui-new/primitives/ChatBoxBase.tsx:144`:

```tsx
// Before:
className="min-h-0 max-h-[min(15rem,20vh)] overflow-y-auto"

// After:
className="min-h-0 max-h-[50vh] overflow-y-auto"
```

**What this does:**
- The old constraint `max-h-[min(15rem,20vh)]` limited the editor to whichever was smaller: 15rem or 20% of viewport height
- The new constraint `max-h-[50vh]` allows the editor to grow up to 50% of the viewport height
- The editor still starts small and only expands as content is added (due to `min-h-0`)
- `overflow-y-auto` ensures scrolling when content exceeds the max height

**Verification:**
- ✅ TypeScript check passes (`pnpm run check`)
- ✅ ESLint passes (`pnpm run lint`)
- The dev server was started and the change was tested in the browser (though the browser MCP encountered an issue during extended testing)

frontend/src/components/ui-new/primitives/ChatBoxBase.tsx

@@ -141,7 +141,7 @@ export function ChatBoxBase({
           onChange={editor.onChange}
           onCmdEnter={onCmdEnter}
           disabled={disabled}
-          className="min-h-0 max-h-[min(15rem,20vh)] overflow-y-auto"
+          className="min-h-0 max-h-[50vh] overflow-y-auto"
           projectId={projectId}
           autoFocus={autoFocus}
           onPasteFiles={onPasteFiles}