Read-only server by default with authenticated write access

Hello

**Summary**
The `cook server` command should operate in **read-only (consultation) mode by default**. Any mutating operation — adding or editing a recipe, modifying the shopping list/cart, or any other state-changing action — must require prior authentication. The authenticated session must persist across browser restarts (no re-login on page refresh or next visit).

**Motivation**
Currently, the server exposes all operations without any access control. This is a problem for users who self-host and want to share their cookbook publicly (read access) while keeping write operations private. A two-tier access model (anonymous read / authenticated write) is the standard pattern for self-hosted tools and would make cook server production-ready for personal or family use.

Operation | Anonymous | Authenticated
-- | -- | --
Browse / view recipes | ✅ | ✅
Search recipes | ✅ | ✅
View shopping list / cart | ✅ | ✅
Add or edit a recipe | ❌ 401 | ✅
Delete a recipe | ❌ 401 | ✅
Add / modify / clear the cart | ❌ 401 | ✅
Any other write operation | ❌ 401 | ✅

_for example_

```toml
[server.auth]
username = "admin"
# bcrypt hash or plain text with a warning
password_hash = "$2b$12$..."
```
and

```bash
# Start with auth enabled (write-protected)
cook server --auth

# Disable auth entirely for local/trusted networks (current behavior, opt-in)
cook server --no-auth
```
Eric