Show upcoming scheduled wakes with live countdowns

## Summary

It would be useful for the agent UI to surface upcoming scheduled wakes, especially self-scheduled sends and cron schedules.

Examples:

- "The agent will wake again in 3:25" with a live countdown for pending self-targeted future sends.
- "Scheduled cron wake: every 5 minutes" and, if feasible, "next wake in 2:17" for cron schedules.

## Motivation

When an agent sends a message to itself with `afterMs`, or has a cron schedule configured, the timeline currently doesn't make the upcoming wake obvious. A small notice would make scheduled autonomous behavior much easier to understand and debug.

## Possible approach

### Future sends

Use manifest schedule rows:

```ts
manifest.kind === 'schedule'
manifest.scheduleType === 'future_send'
manifest.status !== 'sent'
manifest.status !== 'failed'
```

For self-wake language, additionally require:

```ts
manifest.targetUrl === entityUrl
```

Then render a compact notice with a live countdown to `fireAt`:

- `Agent will wake again in 3:25`
- `Agent is waking now…`
- optionally `Scheduled wake overdue` if still pending after `fireAt`

For future sends targeting another entity, use different copy, e.g.:

- `Will send a message to /agent/foo in 3:25`

### Cron schedules

Use manifest schedule rows:

```ts
manifest.kind === 'schedule'
manifest.scheduleType === 'cron'
```

Initial UI could display the cron expression and timezone. A follow-up could compute the next fire time client-side if the existing runtime cron helper is browser-safe/exported, or if we add a small browser-safe helper.

## Placement ideas

- Inline in the timeline as a compact schedule row.
- Entity header/status area as a persistent upcoming wake notice.
- Possibly both: header summary + timeline details.

## Notes

This should be a separate PR from the self-send wake rendering bug fix. That PR makes fired self-send wakes readable; this issue is about making upcoming scheduled wakes visible before they happen.