Feature request: Offline calculator mode for BIP39 mnemonic, xpub, and SeedQR

# Feature request: Offline calculator mode for BIP39 mnemonic, xpub, and SeedQR

I would like to propose an offline **calculator mode** for Jade.

I tested the following setup with an uninitialized Jade DIY:

1. Connected the Jade to a phone charger and started the device.
2. Selected **Options → Temporary Signer**.
3. Entered the first 11 words of a BIP39 mnemonic.
4. Selected **Calculate** so Jade could calculate the valid final word containing the remaining entropy bits and BIP39 checksum.

At that point, my goal was to use Jade to obtain the **xpub** and **SeedQR** corresponding to that BIP39 mnemonic.

However, Jade then asked me to connect to the companion app.

## Feature request

It would be useful to have a completely standalone mode where Jade could act as an offline BIP39 calculator/key derivation device without requiring a companion app.

The workflow could be:

1. User provides the entropy by entering the first 11 or 23 BIP39 words.
2. Jade calculates the valid final word(s), including the checksum and remaining entropy bits.
3. User selects the final word.
4. Optionally enter a BIP39 passphrase.
5. Jade derives and displays the corresponding xpub.
6. Jade displays the SeedQR.
7. Powering off the device destroys the temporary signer state.

The important property would be that this entire process could happen **without Jade ever connecting to a computer, phone, network, or the Internet, and without permanently storing the mnemonic or passphrase**.

Jade would simply use the firmware already installed on the device. All wallet entropy would be supplied externally by the user.

This would create an interesting setup for extremely paranoid/offline users:

**user-generated entropy → Jade verifies/calculates BIP39 → derives wallet information → exports xpub/SeedQR → power off → state disappears**

No companion software would be required for the process.

The remaining trust assumptions would primarily be the Jade hardware and the firmware running on it. A user concerned about those risks could independently verify the firmware, build/flash Jade themselves, and generate all entropy externally.

This would make Jade useful not just as a hardware signer, but also as a completely offline, stateless BIP39 calculation and wallet-export tool.

Start by reviewing the Options → Temporary Signer flow and the Calculate step described in the issue. Define how the standalone workflow would handle final-word calculation, optional passphrases, xpub and SeedQR output, and temporary state destruction without a companion app; completion should cover the full offline workflow.

Written by the indexing model from the issue text.