feat: add paper wallet functionality

[detherminal]

• Adds functionality to import paper wallets by scanning QRs, in the right top part of the coin selection menu (as discussed with rehrar).
• Adds respective functions to each coin for specific functionality.
• Imports the paper wallets automatically.
• Adds Monero gift wallet functionality by scanning and checking out the balance to a new secure wallet if the parameter txid is provided.

[sneurlax]

See notes:

• scanning should respect user's Tor preferences and use Tor if enabled and connected,
• scanning should respect user's node preferences and use preferred nodes other than the default if selected,
• the wallet is swept without confirmation from the user. no txs should be made nor wallets deleted without user confirmation,
• the loading indicator should match the widgets and pattern used elsewhere in the app

[julian-CStack]

@sneurlax may have covered some of these points:

• The wallet creation logic is in a coin metadata class, which doesn’t feel like the right place for it—it makes the structure harder to follow.
• It’s using some custom loading spinner inside a bottom sheet, instead of the animated Stack Wallet loader we use everywhere else. Not sure why that change was made.
• WidgetRef is being passed around outside of a widget, which goes against typical usage and could lead to problems. (It's possible this is done already in other places but I would like to avoid it going forward).
• Monero scans for transactions without respecting Tor settings,.
• The temporary Monero wallet isn’t being closed or deleted on successful import and transfer. It will get deleted but only on next app restart and will take up resources until then.
• There’s also no real error or failure handling—no dialogs or user feedback if something goes wrong. (Another thing we have to work on in other areas of the codebase)

[detherminal]

Added a dialog box for both mnemonic display and verify for the new wallet. Fixed other things as Tor, node choice and loading screen.

[detherminal]

@julian-CStack @sneurlax

[julian-CStack]

I'm probably missing some things as its been a long day

[detherminal]

Moved the logic to a standalone function until wallet creation service is implemented.
Refactored SnackBar into app-wide FlushBar.
Added more error handling and cleaned.

[detherminal]

@julian-CStack @sneurlax Sorry for ping.

[julian-CStack]

While there have been improvements since the opening of this PR, the UX is still not great. The lack of error/failure messages and processing happening without indication in the UI as well as inconsistent mnemonic verification (ui and flow) and wallet creation flow still requires some work. So for now I will merge this into a separate branch until we have time to go over it again. You are welcome to continue working on it and create another PR with changes to https://github.com/cypherstack/stack_wallet/tree/paper-wallet-import