🤖 Allow interrupt (Ctrl+C) in ChatInput when no text selected (#405)

## Problem

Currently, `Ctrl+C` (or `Cmd+C` on Mac) to interrupt the stream only
works when focus is outside editable elements. This means users cannot
interrupt while ChatInput is focused, requiring them to click elsewhere
first.

## Solution

Modified the interrupt handler in `useAIViewKeybinds` to check for text
selection:

- **No selection** → Interrupt stream (allows interrupting from
ChatInput)
- **Has selection** → Browser handles copy (preserves existing behavior)

The logic now allows interrupt in editable elements as long as there's
no active text selection, preventing conflict with the copy keybind.

## Changes

- Updated `useAIViewKeybinds.ts` interrupt handler to check
`window.getSelection()`
- When in editable element with no selection: prevent default and
interrupt
- When in editable element with selection: let browser handle copy

## Testing

Manual testing scenarios:
- ChatInput focused, no selection, Ctrl+C → should interrupt ✅
- ChatInput focused, text selected, Ctrl+C → should copy text ✅
- Other input focused, no selection, Ctrl+C → should interrupt ✅
- Not in input, Ctrl+C → should interrupt ✅

_Generated with `cmux`_

Author: ammar-agent

src/hooks/useAIViewKeybinds.ts

@@ -67,15 +67,32 @@ export function useAIViewKeybinds({
         }
 
         // Normal stream interrupt (non-compaction)
-        // Don't intercept if user is typing in an input field
-        if (!isEditableElement(e.target) && (canInterrupt || showRetryBarrier)) {
+        // Allow interrupt in editable elements if there's no text selection
+        // This way Ctrl+C works for both copy (when text is selected) and interrupt (when not)
+        const inEditableElement = isEditableElement(e.target);
+        let hasSelection = false;
+
+        if (inEditableElement) {
+          // For input/textarea elements, check selectionStart/selectionEnd
+          // (window.getSelection() doesn't work for form elements)
+          const target = e.target as HTMLInputElement | HTMLTextAreaElement;
+          hasSelection =
+            typeof target.selectionStart === "number" &&
+            typeof target.selectionEnd === "number" &&
+            target.selectionStart !== target.selectionEnd;
+        } else {
+          // For contentEditable and other elements, use window.getSelection()
+          hasSelection = (window.getSelection()?.toString().length ?? 0) > 0;
+        }
+
+        if ((canInterrupt || showRetryBarrier) && (!inEditableElement || !hasSelection)) {
           e.preventDefault();
           setAutoRetry(false); // User explicitly stopped - don't auto-retry
           void window.api.workspace.interruptStream(workspaceId);
           return;
         }
 
-        // Let browser handle Ctrl+C (copy) in editable elements
+        // Let browser handle Ctrl+C (copy) when there's a selection
         return;
       }