fix(chat): prevent double send on mobile by removing redundant submit handlers (#719)

PromptInputSubmit already has type="submit" via the parent form, so the
button's click triggers handleSubmit through the form's onSubmit path.
The added onMouseDown/onTouchStart handlers created two extra paths that
both invoked handleSubmit; on iOS Safari a single tap could fire both
touchstart and a synthetic mousedown before isLoading state propagated,
producing two messages and two image-upload roundtrips.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/chat/view/subcomponents/ChatComposer.tsx

@@ -401,14 +401,6 @@ export default function ChatComposer({
             <PromptInputSubmit
               disabled={!input.trim() || isLoading}
               className="h-10 w-10 sm:h-10 sm:w-10"
-              onMouseDown={(event) => {
-                event.preventDefault();
-                onSubmit(event as unknown as MouseEvent<HTMLButtonElement>);
-              }}
-              onTouchStart={(event) => {
-                event.preventDefault();
-                onSubmit(event as unknown as TouchEvent<HTMLButtonElement>);
-              }}
             />
           </div>
         </PromptInputFooter>