Currently the wallet enforces a minimum fee rate of 1 sat/vbyte when creating transactions. Users should be able to set fee rates below 1 sat/vbyte (e.g. 0.5 sat/vbyte) to take advantage of low-fee mempool conditions and save on transaction costs. Problem is we currently round down this amount from 0.5 to 0 so this creates issues...

User stories

• As a user, I want to set a fee rate below 1 sat/vbyte when the mempool is empty, so I can minimize my transaction fees
• As a user, I want to be informed that sub-1 sat/vbyte transactions may take longer to confirm, so I can make an informed choice

Acceptance criteria

• Remove or lower the minimum fee rate floor to allow sub-1 sat/vbyte values
• Allow fractional sat/vbyte input (e.g. 0.1, 0.5)
• Display a warning/info note when the user selects a fee rate below 1 sat/vbyte indicating confirmation may be slower
• Ensure the fee rate input validation accepts decimal values
• Verify the underlying BDK/wallet library supports sub-1 sat/vbyte fee rates

Test cases

• User can enter 0.5 sat/vbyte and successfully create a transaction
• User sees informational note when fee rate < 1 sat/vbyte
• Transaction broadcasts successfully with sub-1 sat/vbyte fee
• Fee estimator suggestions still work correctly alongside manual sub-1 input