Add Multisig announcement modal

[onmax]

Summary

Adds a one-time announcement modal for Nimiq Multisig wallets.

Changes

• Added MultisigAnnouncementModal component with mobile and desktop responsive design
• Added multisig logo SVG and hero image assets
• Integrated modal into app initialization flow (shows once per user)
• Modal opens https://multisig.nimiq.com when button is clicked

TODO

• Update blog post URL
• Sync translations (i18n)

Screenshots

Desktop

Mobile

User Experience

New Users

New users will see this announcement modal once after completing their wallet setup, introducing them to the Multisig feature.

Current Users

Existing users will see this modal once on their next visit to the wallet, as the "shown once" state is tracked per browser via localStorage.

Implementation

• Follows existing modal patterns (PolygonActivationModal, BtcActivationModal)
• Uses localStorage to track "shown once" state
• Shows after USDC activation check, before WelcomeStaking modal
• Clean, minimal code with comments only for "why", not "what"

Testing

Run dev servers for wallet, hub, and keyguard to test the modal flow.

[mraveux]

I pushed some commits but now I think it's good to go, if you're ok with the changes. 👍
Translations have also been synced and we're now only missing the blog URL and https://multisig.nimiq.com/ to be online (since it seems to be unreachable for now).
I will approve the PR but please only merge it once those 2 things are taken care of.
And let me know if / when you need a deployment, I can also do it for you. 👍

[onmax]

Please feel free to merge and deploy it whenever it is ready.

Thanks!