[FEATURE] Saved searches in Search screen

## Summary

Add a **Saved searches** dropdown and a **"Save current search"** button to the Search screen, backed by the new `/api/searches` CRUD endpoints on the server.

> **Depends on:** [`[FEATURE] Saved searches`](https://github.com/guibranco/logstream-server/issues) in `guibranco/logstream-server`

## Motivation

Power users re-apply the same filter combinations daily. Saved searches eliminate repetitive manual filter entry and allow sharing useful search configurations across team members.

## Proposed layout

```
┌─────────────────────────────────────────────────────────────────┐
│ Search [Saved searches ▾] [Save search 🔖]│
├─────────────────────────────────────────────────────────────────┤
│ App key: [__________] Level: [______▾] Date: [____] → [____] │
│ Message: [__________________________________________] │
│ [Clear] [Search] │
└─────────────────────────────────────────────────────────────────┘
```

### Saved searches dropdown

Lists all saved searches. Clicking one loads its filters into the search panel and runs the search immediately.

```
┌─────────────────────────────────┐
│ Production errors today │
│ Billing debug — last hour │
│ PII pattern matches │
│ ───────────────────────── │
│ Manage saved searches… │
└─────────────────────────────────┘
```

### "Save current search" flow

Clicking the 🔖 button opens a small inline popover:

```
┌──────────────────────────────────────────┐
│ Save current search │
│ │
│ Name: [Production errors today ] │
│ │
│ [Cancel] [Save] │
└──────────────────────────────────────────┘
```

On save, the new search appears at the top of the dropdown immediately.

### Manage saved searches screen (or modal)

Accessible from the dropdown footer: full list with edit (rename) and delete per row.

## Files to create / modify

- `src/hooks/useSavedSearches.ts` — CRUD hooks for `/api/searches`
- `src/components/search/SavedSearchesDropdown.tsx` — dropdown component
- `src/components/search/SaveSearchPopover.tsx` — inline save form
- `src/components/search/ManageSavedSearches.tsx` — management list (modal or separate screen)
- `src/screens/SearchScreen.tsx` — integrate dropdown and save button into toolbar

## UX details

- Relative date values (`today`, `-1h`, `-24h`) saved as-is and re-evaluated at execution time — the label "Production errors today" always means today, not the day the search was saved
- When a saved search is loaded, the URL updates so the state is shareable
- Duplicate name: inline error *"A search named 'X' already exists"* (409 from server)

## Acceptance criteria

- [ ] Saved searches dropdown in Search toolbar
- [ ] Clicking a saved search loads filters and runs the search
- [ ] "Save current search" popover with name field
- [ ] Saved search persisted via `POST /api/searches`
- [ ] Manage saved searches: rename and delete
- [ ] Duplicate name handled gracefully (409 error shown inline)
- [ ] Relative date values preserved correctly
- [ ] Dropdown empty state: *"No saved searches yet"*

Start by reading src/screens/SearchScreen.tsx and the requested hook and search components, then verify the server issue's /api/searches CRUD contract. Done means the toolbar, save popover, dropdown, and management view support loading, creating, renaming, deleting, duplicate-name errors, empty state, URL updates, and preservation of relative dates.

Written by the indexing model from the issue text.