Update frontmatter descriptions across Embedded Wallets content

## Problem

Over 120 files in `embedded-wallets/` have `description` frontmatter that doesn't meet editorial
standards (see `.cursor/rules/markdown-formatting.mdc`):

1. **Uses "Web3Auth" instead of "Embedded Wallets"** in the description field. Per
`product-embedded-wallets.mdc`, the product name in prose and metadata should be
"Embedded Wallets" (not "Web3Auth").

2. **Pipe-delimited keyword strings instead of SEO sentences.** Most descriptions follow the
pattern `'Web3Auth SDK - | Embedded Wallets'`. The style guide requires
a single descriptive sentence for SEO and previews.

3. **Redundant `sidebar_label`** on some pages where the value duplicates what the title already
provides.

### Example (current)

```yaml
description: 'Web3Auth iOS SDK - Using Custom Authentication | Embedded Wallets'
```

### Example (expected)

```yaml
description: Authenticate users with your own identity provider using the Embedded Wallets iOS SDK.
```

## Scope

All `.mdx` files under `embedded-wallets/` with a `description` field referencing "Web3Auth."
This includes:

- `sdk/` (iOS, Android, React, React Native, Vue, JS, Flutter, Unity, Unreal, Node) — ~100 files
- `dashboard/` — ~8 files
- `infrastructure/` — 3 files
- `connect-blockchain/` — 1 file

## Suggested approach

1. Rewrite each `description` as a single sentence that uses "Embedded Wallets" as the product
name, describes what the page covers, and works as an SEO snippet.
2. Remove `sidebar_label` where it duplicates the `title`.
3. Validate that no page has both a `title` frontmatter field and a duplicate `# Heading`.

Read `.cursor/rules/markdown-formatting.mdc` and `product-embedded-wallets.mdc`, then inspect the `.mdx` files under `embedded-wallets/`, especially `sdk/`, `dashboard/`, `infrastructure/`, and `connect-blockchain/`. Rewrite each affected description as one SEO-friendly sentence using “Embedded Wallets,” remove duplicate `sidebar_label` values, and verify that no page has both a frontmatter `title` and duplicate `# Heading`.

Written by the indexing model from the issue text.