Add dark mode

[emilghittasv]

• Added CSS custom property tokens for dark mode in _project-theme.scss: page background, card backgrounds, text, borders, shadows, form fields, and interactive states
• All dark overrides follow a consistent three-rule pattern:
  @media (prefers-color-scheme: dark) { ... }  ← OS preference
  [data-theme="dark"] { ... }                  ← manual toggle
  [data-theme="light"] { ... }                 ← override OS dark → light
  -Added a theme toggle button to the top navbar via Alpine.js, with a cookie-backed preference that is pre-read server-side in base.html to eliminate the flash-of-wrong-theme on page load

[denyshon]

We might also want to add dark versions of images in the gallery. What I was thinking of was storing two images under the same entity (light/dark), but there may be different implementations... Anyway, treat this like a note for the future, since this definitely shouldn't be a blocker