[FEATURE] Client health indicators in Applications screen

## Summary

Show real-time health status (active / silent / never seen) and last-seen timestamp for each registered client in the Applications screen, surfacing dead or silent clients immediately.

> **Depends on:** [`[FEATURE] Client health monitoring and last-seen tracking`](https://github.com/guibranco/logstream-server/issues) in `guibranco/logstream-server`

## Motivation

Operators currently have no way to tell from the UI whether a registered application is actively sending logs or has gone silent. This is especially important for detecting failed deployments or misconfigured applications.

## Proposed changes

### ClientTable — updated columns

| Column | Before | After |
|--------|--------|-------|
| **Status** | Active / Inactive pill | Active / Silent / Never seen pill |
| **Last seen** | — (new column) | "3 min ago" / "2 hours ago" / "Never" |

Status pill colours:
- 🟢 **Active** — last seen within the heartbeat timeout
- 🟡 **Silent** — last seen, but exceeded the heartbeat timeout
- ⚫ **Never seen** — `last_seen_at` is null (registered but never ingested)
- 🔴 **Inactive** — `active: false` (manually disabled)

### Client detail drawer — health section

In the edit drawer, below the Active toggle:

```
── Health monitoring ─────────────────────────
Last seen: 3 minutes ago
Heartbeat timeout: [10] minutes
Alert if silent longer than this.
Leave blank to disable monitoring.
──────────────────────────────────────────────
```

### Tooltip on Silent status pill

```
Last seen: 2026-04-17 14:22:00
Silence threshold: 10 minutes
Silent for: 47 minutes
```

## Files to modify / create

- `src/components/clients/ClientTable.tsx` — Last seen column, updated Status pill
- `src/components/clients/ClientStatusPill.tsx` — new component with active/silent/never-seen/inactive states
- `src/components/clients/ClientForm.tsx` — heartbeat timeout field in health section
- `src/hooks/useClients.ts` — include `last_seen_at`, `status`, `heartbeat_timeout_min` in responses

## UX details

- "Last seen" displays as a relative time ("3 min ago") with the absolute timestamp in a tooltip
- Relative time auto-updates every 60 seconds without a server refetch
- The client list auto-refreshes every 30 seconds to pick up new `last_seen_at` values
- Silent clients appear first in the table (sorted by urgency: inactive → silent → never_seen → active)

## Acceptance criteria

- [ ] Last seen column added to client table
- [ ] Status pill shows Active / Silent / Never seen / Inactive states with correct colours
- [ ] Tooltip on Silent pill shows last seen timestamp, threshold, and silence duration
- [ ] Heartbeat timeout field in edit drawer
- [ ] Relative time auto-updates every 60 s
- [ ] Client list auto-refreshes every 30 s
- [ ] Silent clients sorted to top of table

Start with src/components/clients/ClientTable.tsx, ClientStatusPill.tsx, ClientForm.tsx, and src/hooks/useClients.ts to trace the current client data and status handling. Check how the dependent server issue exposes last_seen_at, status, and heartbeat_timeout_min before wiring the listed UI changes. Done means all acceptance criteria pass, including tooltips, relative-time updates, polling, and urgency sorting.

Written by the indexing model from the issue text.