Default Fee in Settings

[i5hi]

Allow users to set a default fee rate in Settings, used as the pre-selected value in all send flows.

User stories

• As a user who consistently prefers a specific fee strategy, I want to set a default fee rate once, so I don't have to adjust it on every transaction

Acceptance criteria

• Default fee rate setting available in Settings
• Options: economy, standard, priority, or custom sats/vB value
• Send flow pre-selects the user's default fee rate
• User can still override per transaction
• Setting persists across app restarts

Test cases

• Set default fee to "economy" in Settings, open send flow, and verify economy is pre-selected
• Set a custom sats/vB value, open send flow, and verify the custom rate is pre-filled
• Override the default in a send flow and verify the override applies only to that transaction
• Restart app and verify the default fee setting persists
• Verify default fee applies to both Bitcoin and Liquid sends

[BullishNode]

Should also have a config "allow sub-1 sats/vbyte" which is default true but can be turned off which changes the mempool api fee recommendation endpoint.

This because somepeople with their own electrum servers would not be able to broadcast below 1 sat/vbyte

[BullishNode]

Should also have a config "allow sub-1 sats/vbyte" which is default true but can be turned off which changes the mempool api fee recommendation endpoint.

This because somepeople with their own electrum servers would not be able to broadcast below 1 sat/vbyte

[thibistaken]

Local branch, need PR to be submitted.

[thibistaken]

Can you submit your PR for review @basantagoswami or are you missing anything?