fix(add-expense): UI/UX improvements for Add screen — closes #5

[ChFardeelAzhar]

Summary

Resolves #5 — UI/UX Enhancement: Add Screen Design Update

Visual Changes

Before	After
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Changes Made

UI/UX (visible):

• Button color updated to match app teal theme
• Input field borders now use teal on focus instead of black
• Field labels changed to sentence case (Material3 guideline)
• Improved spacing between form fields

Bug fixes (same PR — found during UI work):

• Fixed date defaulting to Jan 01, 1970 — was initialized as 0L, now null
• Fixed Cancel button in DatePicker incorrectly saving date instead of dismissing
• Added amount validation — empty/zero amount no longer saves silently
• Fixed dropdown not closing on outside tap (onDismissRequest was empty lambda)
• Replaced hardcoded Color.Black with MaterialTheme.colorScheme tokens for dark mode support

Testing

• Add expense flow works end to end
• Date picker cancel dismisses correctly
• Empty amount shows error message
• Tested in both light and dark mode