feat: add theme change button

[kingazm]

📄 Pull Request Description

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🧩 What was changed?

Added a light/dark theme toggle button to the top bar, placed directly to the left of the existing contrast slider button.

• app/components/shared/top_bar.tsx - new toggle button + isDark state and effect
• app/tailwind_styles.css - switched dark mode from media-query to class-based
• app/root.tsx - added inline script for flash-free theme initialization
• app/i18n.ts - added themeToggle translation key (EN + PL)

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💡 Why was it changed?

Previously, dark mode was controlled entirely by the OS/browser prefers-color-scheme setting with
no way to override it from within the app. Users who want dark mode on a light-mode system (or
vice versa) had no way to switch. This brings the theme control in-app, consistent with how the
contrast and language settings already work. In addition, this was also project's supervisor suggestion.

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⚙️ How was it implemented?

• isDark is initialised from localStorage('jucaneat-theme'), falling back to
  window.matchMedia('(prefers-color-scheme: dark)') so first-time visitors get the theme
  automatically.
• A useEffect toggles the dark class on and persists the choice to localStorage
  on every change.
• Flash prevention: An inline <script> in root.tsx runs synchronously before the renders,
  applying the dark class from storage before React hydrates. No flash of wrong theme on hard
  reload.
• Added tailwind @variant dark (&:where(.dark, .dark *)); to tailwind_styles.css to switch from
  the default @media (prefers-color-scheme) variant to a class-based one, so the button actually
  controls all dark: utility classes.
• Button: Uses the same xsm size and border/bg styling as the EN/PL and contrast buttons. Shows
  Moon icon in light mode and Sun icon in dark mode (lucide-react, already a project dependency).

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⚠️ Side Effects or Risks

• Behaviour change for existing users: Dark mode now requires the dark class on instead of
  the OS media query. On first load, the inline script mirrors the system preference, so existing
  users will see no change, but if localStorage ever has a stale value it will override the system
  setting.
• Classes in the app now respond to the class rather than the media query. Any component
  that relied on automatic OS-driven dark mode will now only switch when the user explicitly
  toggles or when the inline script fires on load. This is intentional but is a global behaviour
  change.

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✅ Checklist

• All 4 sections above are clearly filled out
• Tests and documentation updated N/A, tested manually