Consolidate duplicated infrastructure content in dapp-share.mdx

## Problem

`embedded-wallets/sdk/ios/advanced/dapp-share.mdx` duplicates content from the [Embedded Wallets infrastructure](/embedded-wallets/infrastructure/) page across two sections:

- **"Embedded Wallets infrastructure at a glance" (lines 7-28)** reproduces the share-splitting explanation (ShareA, ShareB, ShareC, 2-of-3 reconstruction) that already lives on the infrastructure page.
- **"Dapp share in iOS" (lines 49-53)** repeats the dapp share definition nearly verbatim from the end of the "Mobile platform user experience" section (lines 45-47).

The "Mobile platform user experience" section (lines 30-47) is also largely background explanation that could be condensed.

## Suggested fix

1. Replace the "Embedded Wallets infrastructure at a glance" section with a 1-2 sentence summary and a link to the infrastructure page.
2. Condense "Mobile platform user experience" to 2-3 sentences covering the key point: on mobile, browser storage is unreliable because users may unknowingly clear it, so a dapp share provides a backup.
3. Remove the duplicated definition in "Dapp share in iOS" (lines 51-53) since it repeats lines 45-47.

## Files affected

- `embedded-wallets/sdk/ios/advanced/dapp-share.mdx`

Start with embedded-wallets/sdk/ios/advanced/dapp-share.mdx and compare its duplicated sections with the linked Embedded Wallets infrastructure page. Condense the infrastructure and mobile user-experience explanations, then remove the repeated dapp share definition. Done means the page preserves the key mobile backup rationale without duplicating existing documentation.

Written by the indexing model from the issue text.