feat: add pink-purple theme, fix image paste race condition, allow @/commands anywhere in input

- Add Pink Purple theme (hot pink/purple/magenta on dark plum background)
- Fix race condition where clearPastedImages() in input-area ran before
  the async message handler could read the images, silently dropping them
- Allow @ file picker and / command menu to trigger at any cursor position,
  not just when the input is empty
- Update CHANGELOG and README with new changes

src/interfaces/StreamCallbacksWithState.ts

@@ -7,4 +7,5 @@ import type { StreamCallbacks } from "@/types/streaming";
 export interface StreamCallbacksWithState {
   callbacks: StreamCallbacks;
   hasReceivedContent: () => boolean;
+  hasReceivedUsage: () => boolean;
 }