[Enhancement] Add dark mode support

[niat-web]

Feature Request

What:
Add the ability to switch between
light mode and dark mode.

Expected behavior:

• Toggle button (🌙/☀️) in the top navbar
• Dark mode: dark background, light text
• Light mode: current design (default)
• User preference saved in localStorage
• On first visit, follows system preference
  (dark if OS is in dark mode)

Why this is important:
Many users prefer dark mode especially
when working at night.
It also reduces eye strain.

Technical implementation:

• Add CSS variables for colors
• Toggle class on body element
• Save preference in localStorage
• Check system preference on first load

Acceptance Criteria:

• Dark mode toggle button in navbar
• All pages look good in dark mode
• Preference saved across browser sessions
• Follows system preference on first visit