feat: token usage toggle (setting + /usage command) + timestamp fixes

Token usage display:
- Add 'show_token_usage' boolean to settings (default: false, off by default)
- Settings panel: checkbox 'Show token usage after responses'
- /usage slash command: instant toggle with toast feedback, persists to
  server, updates checkbox if settings panel is open, re-renders messages
- Boot: load show_token_usage alongside send_key on startup
- ui.js: gate usage badge on window._showTokenUsage flag

Timestamps:
- streaming.py: stamp 'timestamp' on every message that lacks one at
  conversation completion; old messages (no timestamp field) now get a
  wall-clock time the first time they're touched by a new turn
- messages.js: stamp _ts on the last assistant message at done-event time
  so the time shows immediately on the current turn before next reload
- Timestamps already render in the UI (Sprint 14): faint time on each
  role header line, full opacity on hover, full date in title tooltip

api/streaming.py

@@ -170,6 +170,11 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
                 persist_user_message=msg_text,
             )
             s.messages = result.get('messages') or s.messages
+            # Stamp 'timestamp' on any messages that don't have one yet
+            _now = time.time()
+            for _m in s.messages:
+                if isinstance(_m, dict) and not _m.get('timestamp') and not _m.get('_ts'):
+                    _m['timestamp'] = int(_now)
             s.title = title_from(s.messages, s.title)
             # Read token/cost usage from the agent object (if available)
             input_tokens = getattr(agent, 'session_prompt_tokens', 0) or 0