feat(tui): delete queued message while editing with ctrl-x / cancel with esc

Today there's no way to remove a queued message — ↑ loads it for edit,
ctrl-K dispatches the head, but a draft you no longer want stays put
forever. ctrl-C just clears the composer and exits edit mode without
touching the queue.

Two new bindings, both gated on queueEditIdx !== null so they're
inert when the user isn't pointing at a queue item:

- ctrl-X — delete the queue item being edited, clear composer, exit
  edit mode.  "cut" matches the mental model and doesn't collide with
  any existing binding.
- esc — cancel the edit (composer clears, item stays in queue).
  Mirrors ctrl-C's existing behavior so muscle memory has two paths.

Header line now reads `queued (3) · editing 2 · ⌃X delete · esc cancel`
when in edit mode, so the affordance is discoverable without /help.
The /help hotkey table also gets a Ctrl+X entry.

ctrl-C is intentionally unchanged: it should never destroy queued
content.  Cancel is non-destructive (esc / ctrl-C); only ctrl-X
removes the item.

ui-tui/src/hooks/useQueue.ts

@@ -1,5 +1,15 @@
 import { useCallback, useRef, useState } from 'react'
 
+export function removeAt<T>(arr: T[], i: number): T[] {
+  if (i < 0 || i >= arr.length) {
+    return arr
+  }
+
+  arr.splice(i, 1)
+
+  return arr
+}
+
 export function useQueue() {
   const queueRef = useRef<string[]>([])
   const [queuedDisplay, setQueuedDisplay] = useState<string[]>([])
@@ -36,6 +46,19 @@ export function useQueue() {
     [syncQueue]
   )
 
+  const removeQ = useCallback(
+    (i: number) => {
+      const before = queueRef.current.length
+
+      removeAt(queueRef.current, i)
+
+      if (queueRef.current.length !== before) {
+        syncQueue()
+      }
+    },
+    [syncQueue]
+  )
+
   return {
     dequeue,
     enqueue,
@@ -43,6 +66,7 @@ export function useQueue() {
     queueEditRef,
     queueRef,
     queuedDisplay,
+    removeQ,
     replaceQ,
     setQueueEdit,
     syncQueue