fix(dashboard): stop mobile send-button double-fire that aborted chats

The ChatView and QuickChat send/stop buttons each had both an
onPointerDown and an onTouchStart handler invoking the action. On a
quick mobile tap both fire, so handleSend / handleSendMessage /
stopStreaming ran twice in rapid succession. The second invocation
closed the first's SSE stream (streamRef.current.close()), the server
treated that as a cancel via beginGeneration, and the chat ended with
no output — exactly matching the reported "tap silently fails, long
press works" symptom (long press happened to suppress one of the two
events).

Both handlers now early-return when the existing handledMobile*Ref
flag is already set, so only the first event for a given tap fires the
action. The send button additionally gets touch-action: manipulation
(removes the click delay that lets the textarea blur win the race) and
an expanded invisible hit area via ::before so slightly-off taps don't
land on the surrounding textarea and dismiss the keyboard without
sending.

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

packages/dashboard/app/components/ChatView.css

@@ -856,6 +856,25 @@
   align-items: center;
   justify-content: center;
   flex-shrink: 0;
+  /* Disable double-tap zoom delay on mobile so the click handler fires
+     immediately on tap, and ignore other gestures the browser would
+     otherwise speculate on. Without this, quick taps can land in the
+     ambiguity window where the browser dismisses the keyboard (textarea
+     blur) before deciding whether to register a click on the button. */
+  touch-action: manipulation;
+}
+
+/* Expand the effective tap target without changing the visual size, so
+   slightly-off taps still land on the button instead of falling through
+   to the surrounding textarea (which would dismiss the mobile keyboard
+   without sending). */
+.chat-input-send::before {
+  content: "";
+  position: absolute;
+  inset: -8px;
+}
+.chat-input-send {
+  position: relative;
 }
 
 .chat-input-send:disabled {