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/interfaces.ts

@@ -125,6 +125,7 @@ export interface ComposerActions {
   handleTextPaste: (event: PasteEvent) => MaybePromise<ComposerPasteResult | null>
   openEditor: () => Promise<void>
   pushHistory: (text: string) => void
+  removeQueue: (index: number) => void
   replaceQueue: (index: number, text: string) => void
   setCompIdx: StateSetter<number>
   setHistoryIdx: StateSetter<null | number>