base.njk nav element missing container class

## Problem

The `base.njk` template wraps the nav in:
```html
<nav class="nav">
```

But the dart_node site uses:
```html
<nav class="nav container">
```

The `container` class is needed for proper max-width and centering. Without it, the nav content stretches to full viewport width.

## Suggestion

Add `container` to the nav class list, or provide a configurable wrapper class.