Add currency and percentage input plugins to mortgage calculator

- Currency inputs auto-format with thousand separators on input
- Only allows numeric characters (strips letters, symbols, etc.)
- .val() returns raw integer for calculations
- Percentage inputs allow only numbers and one decimal point
- Select all on focus for easy replacement
- Maintains cursor position while typing
- Real-time sync between down payment dollar and percent fields

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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