refactor(tui): turn elapsed lives in FaceTicker; emit done-in sys line

Drops `lastUserAt` plumbing and the right-edge idle ticker. Matches the
claude-code / opencode convention: elapsed rides with the busy indicator
(spinner verb), nothing at idle.

- `turnStartedAt` driven by a useEffect on `ui.busy` — stamps on rising
  edge, clears on falling edge. Covers agent turns and !shell alike.
- FaceTicker renders ` · {fmtDuration}` while busy; 1 s clock for the
  counter, existing 2500 ms cycle for face/verb rotation.
- On busy → idle, if the block ran ≥ 1 s, emit a one-shot
  `done in {fmtDuration}` sys line (≡ claude-code's `thought for Ns`).

ui-tui/src/app/interfaces.ts

@@ -237,7 +237,6 @@ export interface SlashHandlerContext {
     newSession: (msg?: string) => void
     resetVisibleHistory: (info?: null | SessionInfo) => void
     resumeById: (id: string) => void
-    setLastUserAt: StateSetter<null | number>
     setSessionStartedAt: StateSetter<number>
   }
   slashFlightRef: MutableRefObject<number>
@@ -300,11 +299,11 @@ export interface AppLayoutProgressProps {
 export interface AppLayoutStatusProps {
   cwdLabel: string
   goodVibesTick: number
-  lastUserAt: null | number
   sessionStartedAt: null | number
   showStickyPrompt: boolean
   statusColor: string
   stickyPrompt: string
+  turnStartedAt: null | number
   voiceLabel: string
 }