chore: add Dependabot config for automated dependency updates

## Problem

The monorepo has no automated dependency management. Several packages are critically outdated:

- Babel v6 (`babel-cli@^6.26.0`, `babel-preset-env@^1.6.1`) — EOL since 2019, no nullish coalescing/optional chaining
- Jest v22 (`jest@^22.4.2`) — 2018 release, missing modern features
- Downshift v1.x (`downshift@^1.31.2`) — current stable is v9, missing 7 years of a11y fixes
- Webpack v4 (`webpack@^4.18.0`) — v5 has been stable since 2020
- Emotion v10 (`@emotion/core@^10.0.28`) — v11 is current (Vue package already uses v11)

## Suggested Fix

Add `.github/dependabot.yml`:

```yaml
version: 2
updates:
- package-ecosystem: 'npm'
directory: '/packages/web'
schedule:
interval: 'weekly'
- package-ecosystem: 'npm'
directory: '/packages/vue'
schedule:
interval: 'weekly'
- package-ecosystem: 'npm'
directory: '/packages/native'
schedule:
interval: 'weekly'
- package-ecosystem: 'npm'
directory: '/packages/reactivecore'
schedule:
interval: 'weekly'
```

Also recommend upgrading to `actions/checkout@v4` and `actions/setup-node@v4` (current is v2).

## Severity

Medium — long-term maintenance burden

Create .github/dependabot.yml using the four npm package directories listed in the issue; first inspect the repository’s package layout and existing GitHub Actions workflow files. Ensure /packages/web, /packages/vue, /packages/native, and /packages/reactivecore receive weekly updates, and update checkout/setup-node references from v2 to v4 where applicable. Done means all four packages are covered and the YAML validates.

Written by the indexing model from the issue text.