Message signing network mismatch bugfix; OptionDisabledView bugfix

[kdmukai]

Message signing was already checking for "Network Mismatch" if a user was configured for MAINNET in Settings but scanned in a signmessage QR with a TESTNET/REGTEST derivation path specified.

However, it was not checking the opposite: Settings configured for TESTNET or REGTEST but scan in a signmessage QR for MAINNET.

Second bug: When message signing is disabled in Settings, the OptionDisabledView appears but then did not return any values, resulting in a second error screen.

This PR:

• Adds that opposite check (Settings configured for TESTNET/REGTEST, signmessage QR for MAINNET).
• Moves the network check to be as early as possible in the flow.
• Updates the test_sign_message_network_mismatch_flow to test for all possible network mismatches and DRY-ifies it for internal reuse.
• Adds option on OptionDisabledView to jump straight to the relevant Settings update.
• Moves message signing-related decision logic out of ScanView and into SeedSignMessageStartView
• Adds OptionDisabledView to screenshot generator.
• Adds test for sign message being disabled.

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[newtonick]

ACK and tested