[BUG]: Cannot import Nostr accounts using private key

**Describe the bug**

Currently OneKey supports creating Nostr key pairs in wallets created in OneKey or derived from BIP-44 mnemonics.

However, it seems OneKey does *not support importing existing Nostr accounts using the private key*.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to 'Import wallet'
2. Click on 'Private key'
3. Open the 'Network' drop-down and type 'Nostr'
4. No Nostr option is found


**Expected behavior**

Nostr should be found as one of the options in the network selection. The screenshot below shows it as an option in OneKey created wallets.


**Desktop (please complete the following information):**
- OS: Linux
- Browser: Chrome
- Version: OneKey extension version 5.5

**Additional context**

While reseaching this I noticed that the blog post https://blog.onekey.so/onekey-now-supports-nostr-protocol-a-new-era-of-decentralized-social-networking-8550a7bc03dc links to two dead links:
* https://help.onekey.so/hc/en-us/articles/8504160917007
* https://help.onekey.so/hc/en-us/articles/8515405561231

You might want to fix these as well.

Start at the Import wallet flow, choosing Private key and inspecting the Network dropdown for how networks are listed and filtered. Add Nostr to the available private-key import options, verify that an existing Nostr account imports successfully, and check the linked blog references separately if documentation cleanup is included.

Written by the indexing model from the issue text.