Send Set Custom Fee

[ovitrif]

Related: #49
Design: Figma v57 Send - Set Custom Fee

This PR adds the feature to set a custom fee rate in satPerVb during the onchain send flow.

Description

• Send custom fee screen UI & logic
• Use pixel perfect sizing for sheet topbar and drag handle
• Fix to navigate back on speed click in settings
• Show spinner while loading fee amount on review screen
• Validate against max and min fee limits and disable invalid speeds
  • Fee must not exceed 50% of available balance and it must be less than the remaining funds after send

Preview

Screen_recording_20250808_003801.mp4

QA Notes

Test Custom Fee Screen:

• test with custom fee already set via settings
• test by setting different custom speeds
• test by switching from fee rate preset to custom
• test validation by using amount close to max

[jvsena42]

I see a tendency for wallets to allow a fee rate smaller than 1. I wonder when we should start thinking about this to Bitkit