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/app/useInputHandlers.ts

@@ -311,6 +311,10 @@ export function useInputHandlers(ctx: InputHandlerContext): InputHandlerResult {
       return clearSelection()
     }
 
+    if (key.escape && cState.queueEditIdx !== null) {
+      return cActions.clearIn()
+    }
+
     if (key.upArrow && !cState.inputBuf.length) {
       const inputSel = getInputSelection()
       const cursor = inputSel && inputSel.start === inputSel.end ? inputSel.start : null
@@ -357,6 +361,11 @@ export function useInputHandlers(ctx: InputHandlerContext): InputHandlerResult {
       }
     }
 
+    if (isCtrl(key, ch, 'x') && cState.queueEditIdx !== null) {
+      cActions.removeQueue(cState.queueEditIdx)
+      return cActions.clearIn()
+    }
+
     if (key.ctrl && ch.toLowerCase() === 'c') {
       if (live.busy && live.sid) {
         return turnController.interruptTurn({