[FEATURE] Dashboard / overview screen

## Summary

Add a **Dashboard** screen as the default landing page, showing an at-a-glance summary of log activity powered by the new `GET /api/stats` endpoint on the server.

> **Depends on:** [`[FEATURE] Metrics and statistics endpoint`](https://github.com/guibranco/logstream-server/issues) in `guibranco/logstream-server`

## Motivation

Currently the app opens directly on the live log stream. New users and on-call engineers need a quick overview — are there more errors than usual? Which application is the noisiest right now? — without wading through raw log entries.

## Proposed layout

```
┌─────────────────────────────────────────────────────────────────┐
│ Dashboard Period: [15m] [1h] [6h] [24h] │
├───────────────────────┬───────────────────────┬─────────────────┤
│ Total entries │ Error rate │ Ingestion rate │
│ 9,342 │ 42 (0.4%) │ 155 / min │
├───────────────────────┴───────────────────────┴─────────────────┤
│ By level (bar chart) │ By source │
│ ████████████ debug 8,000 │ HTTP 4,842 UDP 4,500 │
│ ████ info 1,200 │ │
│ ▌ warning 100 │ Top categories │
│ ▌ error 42 │ payments 980 │
│ │ auth 412 │
│ By application (bar chart) │ jobs 201 │
│ billing-api ████ 3,100 │ │
│ auth-service ████ 7,142 │ │
└────────────────────────────────┴────────────────────────────────┘
```

## Files to create / modify

- `src/hooks/useStats.ts` — `GET /api/stats?period=1h`, queryKey `['stats', period]`, refresh every 30 s
- `src/screens/DashboardScreen.tsx` — assembles all stat cards and charts
- `src/components/dashboard/StatCard.tsx` — single metric tile (value, label, optional delta vs previous period)
- `src/components/dashboard/LevelBarChart.tsx` — horizontal bar chart of entries by level using `recharts`
- `src/components/dashboard/AppKeyChart.tsx` — horizontal bar chart of entries by app_key
- `src/components/dashboard/PeriodSelector.tsx` — pill group for 15m / 1h / 6h / 24h / 7d
- `src/components/dashboard/SourceBadges.tsx` — HTTP vs UDP ingestion counts
- Header navigation — **Dashboard** becomes the default (first) tab

## UX details

- Period selector is sticky — changing it refreshes all cards simultaneously
- Error rate card turns **red** when above 1%, **amber** when above 0.1%
- Auto-refresh every 30 seconds; show a subtle `Last updated: Xs ago` label
- Loading state: skeleton cards matching the layout
- Empty state (no logs yet): friendly onboarding message with a link to the docs

## Tech constraints

- `recharts` (already a dependency) for bar charts
- No new npm dependencies
- TanStack Query v5 for server state
- Tailwind utility classes only

## Acceptance criteria

- [ ] Dashboard is the default landing screen (first tab)
- [ ] Period selector (15m / 1h / 6h / 24h / 7d) filters all cards
- [ ] Stat cards show total, error rate, ingestion rate
- [ ] Level bar chart renders correctly
- [ ] App-key bar chart renders correctly
- [ ] Source badges show HTTP vs UDP split
- [ ] Top categories list rendered
- [ ] Auto-refresh every 30 seconds
- [ ] Loading skeleton shown while fetching
- [ ] Error state handled gracefully (server unreachable)
- [ ] When `GET /api/info` returns `features.retention: false` or `features.client_management: false`, corresponding cards are hidden

Start by inspecting the existing screen and header navigation, then read the proposed src/hooks/useStats.ts and src/screens/DashboardScreen.tsx entry points. Review the dashboard component files and the GET /api/stats contract before implementing. Done means the listed cards, charts, states, period filters, refresh behavior, navigation, and feature-based visibility meet the acceptance criteria.

Written by the indexing model from the issue text.